Falta model 250s are 980 series, the 513 is the later centreport motor that doesn't have a rear transfer port.
Luckily most parts are the same, pistons & rod kits are the common to all but the twinports. Main difference with the Falta & 513 model 250s to earlier 980 models is the internal rotor ignition which use a different sized taper on the crank. Pre73 models also use a taperlock in the primary drive gear instead of a keyway
Clutch parts are common with all 980,981 & 984 motors, the 513 uses a different dry clutch with a large number of small fingers, it can be fitted but needs the primary drive gear & clutch gear as the basket screws to the gear rather than using the special nut to retain it
Parts are extremely easy to get, CRHighperformance is the nearest place in the US & is great to deal with, in the UK CZStuff is good, I also use Etovar.cz for a lot of the smaller parts. There is a company in Poland I've dealt with a few times that has multiple eBay accounts
