From the article I linked above....Were the engines made in house?
"As the Ossa importer, John Taylor was able to use his connections at Ossa to have them modify a twin-cylinder 460cc Ossa road race engine, designed by Eduardo Giro, into a 488cc dirt bike engine by mating two 250cc Ossa Pioneer top-ends, including the pistons, cylinders and heads, onto a unified two-cylinder engine case. The engine had a bore and stroke of 72mm by 60mm and two 24mm IRZ 4G dual-needle carbs. The result was an engine that could be used as a twin or a twingle."