All of there are no longer available new.
FC540V**-** is a 17hp OHV 535cc Stroker
FC420V**-** is a 14hp OHV 420cc
FB460V**-** is a 12.5hp Flat Head Valve in Block 460cc
I have all the factory service manuals for these.
The starters are High Torque Nippondenso....they very rarely fail except for the coil (again, on rare occasions.)
They are quite cheap and easy to find used on ebay. New, they are a shocking $475.00 or more.
The only performance parts available are for the 460, and that are a billet flywheel and I have billet connecting rods, In which is probably not needed, I have never seen a stock 460 rod break on it's own. There has been talk about a billet flywheel for a 540, but that's it...just talk.
All are contenders for a governed class. These three are practically bullet proof, they all use a real world pressurized oil pump
that pumps thru the crank and well as the entire engine, Breather is very well thought out too.
The only failures I have seen are from changing the oil once every 5 years, or lack there of. I just picked up a 540 engine that was seriously cooked, it was overheated for a couple seasons, Mice made a couple nests blocking the air flow in the cooling fins. The piston came apart but did not chew up the cylinder walls...amazing.