I'm told this is a Colin Lyster frame- for engines unknown. The Rhodesian ex-racer made his name as a motorcycle innovator in Europe in the sixties, he pioneered the use of hydraulic disc brakes for motorcycles. He went on to design frames for racing motorcycles as well as developing engines to replace the long-in-the-tooth British singles. Below, a nice Norton twin cafe racer.
Classic Bike Magazine of July 1999 asks what make of frame this might be. I think the Yamaha 2 stroke twin is a good guess and the details do indicate Lyster.