Aye Loco,
SNAP!

Rebuilding an XT600E motor at present, along with others and starting an extension!

Megacycle cam from USA is a common mod:
Megacycle_catalogueAlternative supplier, again USA:
ThumperstuffOr speak with Martin Sweet at Slipstream, with years of experience with these motors in Mono racing etc. He has just rebuilt my crank for me

:
Slipstream_tuningFor a carb, Allen's would be a good starter:
Allen's_Performance_carbsI suspect that SteveH, andy230 and SteveL may also contribute to this posting, with their pennyworth and you could try using the 'Search' box in the top right of the screen to search the various boards in this forum for further information. "SRX camshaft" and "SRX carb" would be good starting searches.
Hopefully, this helps and if you have any specifics, post them up and we will try to point you in a direction!

It's up to you to decide whether you like the direction!

My regards, Bill