One Speeds ne-speed bikes are very easy to convert, since they usually already have 110 mm spacing . Some of the better-quality cruisers work OK as fixed gears, but watch out for low bottom brackets. One of my favorite bikes is my 1916 Mead Ranger . This was originally built as a coaster brake bike with 28 inch single-tube tires on wood rims. It's surprisingly pleasant to ride.
Mountain bikes tend to have adequate ground clearance, although most, alas, have vertical dropouts .
I have set up a couple of mountain bikes with flip-flop hubs , so that I get a fixed gear on one side and two different freewheel gears on the other. This is done with a double chainwheel and a two-speed freewheel. Bruce Ingle , a fellow member of the Charles River Wheelmen , has gone me one better, and made a triple-fixed mountain bike. He used a Shimano cassette hub , which he immobilized by brazing the ratchet mechanism together. I am a bit nervous as to the long-term prospects for this hub, in particular the connection between the freehub body and the hub shell, but I think I will have to copy his setup.