Does anyone know a quote, from an opponent of MLK (e.g. a sheriff) which presents him as radical?

If by as a radical, you mean as someone who declares himself of the left, as the term is generally understood, King, himself was not; he had, as the King Institute at Stanford University points out:

to defend himself against allegations of being a Communist, though his view that “Communism and Christianity are fundamentally incompatible” did not change.

Given Marxism was predicated on a materialist philosophy on the nature and progress of history and he was a Christian, it's hard to see how anyone could mistake him for a communist. His 'radicalism' came from taking his Christianity seriously. Nevertheless, given his commitment to speaking up against injustice, he was sympathetic towards it, decrying:

America’s “morbid fear of Communism,” [and argued] that it prevented people from embracing a “revolutionary spirit and … declaring eternal opposition to poverty, racism, and militarism”

One need not, after all, share all the commitments of a philosophy, to find something to admire in it, if there are other important points on which one could agree, and King found plenty to admire in the then communisms revolutionary commitment towards changing working conditions in favour of those who were oppressed by them. It's his commitment to these values that most likely led to his slurs and slanders that the newspapers and media were only too happy to amplify. For example:

The New American has, by its own admission, published many articles on King’s 'communist connections' and in its predecessor magazine, American Opinion, and the monthly bulletins of The John Birch Society, the parent organization of both publications. In fact, in this article they point out that the book It's Very Simple: The True Story of Civil Rights, by Alan Stang, who was a member and writer for the John Birch Society) wrote:

Through Levison’s influence, other subversives were attracted to SCLC [The Southern Christian Leadership Conference, which King helped found]. Hunter Pitts O’Dell, former National Committee member of the CPUSA, was employed by SCLC. In 1962, when King mentioned to Levinson that he was thinking of adding an administrative assistant to his staff, Levinson recommended O’Dell, who was then head of SCLC’s New York office. King said he liked the idea. At that time, King was well aware of Levison’s and O’Dell’s communist affiliations.

Also, Robert Welch, the founder of The John Birch Society, was keen on 'exposing' King’s communist connections. He wrote the following words for May 1968 John Birch Society Bulletin.

During this career of a brief dozen years, [King] was constantly surrounded by, and associated with, Communists. He “employed” such Communists as Bayard Rustin and Hunter Pitts O’Dell; and he worked closely with Carl Braden and James Dumbrowski.

All of these quotes present MLK as a communist radical by association (despite what he himself had to say about it) and it's also remarkable that they say nothing about what these communists had done that constitutes a crime! It seems rather like the John Birch Society thinks that merely being a communist, or being friends with a communist, or associating yourself with one, is a crime in itself: strange in a country that prides itself on free thinking and free speech.

The FBI under J. Edgar Hoover spent a great deal of effort trying to tie MLK to Communism or otherwise discredit him. They even tried to blackmail him into committing suicide (though that is outside the scope of this question). In a DOJ memorandum on July 6, 1965, Hoover discussed whether "there [had] been any hard Communist Party line tying together [opposition to] Vietnam and the civil rights movement," he wrote:

I stated there was no doubt in my mind from information we have had in the past few months that King, Levison and Jones in New York have been having these huddles together meeting at the Kennedy Airport motor inn...Stanley Levison is a member of the Communist Party and Clarence Jones also.

https://alphahistory.com/coldwar/j-edgar-hoover-martin-luther-king-1965/

At the time, of course, Communism was the ultimate expression of radicalism in the USA.

While not the instigator, and presently unable to direct or control the coming Negro August 28 March on Washington, D.C., communist officials are planning to do all possible to advance communist aims in a supporting role. Martin Luther King, a key figure in the March, does have as an advisor, Stanley Levinson, a secret Communist Party member King himself has been reported to be a Marxist.

It should be very clear to all of us that Martin Luther King· must, at some propitious point in the future, be revealed to the people of this country and to his Negro followers as being what he actually is: a fraud, demagogue, and moral scoundrel.

We do not have, and no Government agency or private organization has, any yardstick which can accurately measure "influence", in this particular context; even when we know it does exist such as Levinson over Martin Luther King and King's influence over other Negro leaders. Personally, I believe in the light of of King's powerful demagogic speech yesterday he stands head and shoulders over all other Negro leaders put together when it comes to influencing great masses of Negroes.

We must mark him new, if we have not done so before, as the most dangerous Negro of the future in this Nation from the standpoint of communism, the Negro and national security.

–– FBI Documents Related to its Plans to "Neutralize" Martin Luther King by U.S. Federal Bureau of Investigation (FBI)

In this case William Sullivan, the FBI’s director of intelligence wrote that. The demagogic speech was "I have a dream".