Gday Canolman
go for a 13:1 644 dude!
im just using a prox "650" piston (elko copy) normally the CR with the 80mm stroke would be 11.8:1 like stock but my block and head have been machined both by me and other POs the CR is about 12.6 :1
becasue of that teh cam chain was too short so i had to move the cam up 1mm then had to grind clearance out of the rocker arms for the cam lobes, spanner made a special cam chain tensioner for his bike, FD racing uses 2 adjustable tensioners one at the front and one at the back (no headgasket)
look up the specs on a wossner 8583 just need to pop it in a lathe and take some meat out of it to get what you want.