Honestly, if it were me, and parts are hard to get as it sounds, I'd stick with a nice stock stroke 350.
But if you want gobs and gobs of power for a play bike, it's hard to beat a 421 Serval.
Not trying to derail a sale, it's just my opinion on the matter.
You need to decide the primary use of the quad and pick an engine from there.
Are you against sending the motor to the states and having it gone through? If it's fairly new, you probably have a bunch of bores left and could have a nice stock stroke 350 done .
Just options.