Welcome, and congrats....on the pilot...not the engine trouble
You've got a rare bird there, there's not very many 400 jugs bored to 500 engines around.
Dennis had one...just sold it I think....did you buy from him? And someone else just got one not too long ago....Odypilots?
I have two 500 engines, but they have the bottom ends modified for CR500 top ends....so I dunno much about the LRD setup, sorry.
I've heard of cool heads, I don't have contact info for them right off....someone here should..PolitHawk?? (link king LOL)
Personally, first I would decide whether you want to be running race fuel or not. I got both mine set for pump fuel, I didn't want to have to screw with race fuel every time I wanted to ride.
Then decide on whether to keep that 500 setup or not. I would get ahold of someone else who has one and talk to them about their experiences with them, and find out where you can get the parts you need for it...guess head gasket is the only issue?....what do they use for pistons...CR500? It's easy to swap back to a OEM 400 cylinder....so that's always an option.
I'm not trying to steer you away from the LRD 500 setup. But for me, I'd want more info on them before I decide to stay with that setup.
I'm selling my runinning CR500 engine, the spare CR500 engine that's in pieces and a spare 400 engine in pieces (almost all there) LOL there's some more options for ya