Well i know shit. but i have learnt a few things, they are all similar frames but the donks change depending on era, and certain motrs dominated for a period, bit it weslake, godden, jawa and so on, the main differances seems to be in the valves 2 to 4, upright ( moderns are laydown) and the oil pumps out to waste, or the 898 which recicle back to engine. Magneto - Black boxes
other than that there isnt to much to say. there all kinda just a set of cases with a crank a piston and a head, the rest seems to be period based some went better than other at the period.
i found a really good web site that explained all this ill try find the link, it all made perfect sence after you read it.