diameter = sqrt( disp ) x 2. sqrt being square root. that's a 31ci as I recall, so 507cc give or take. rounding down to 500 (close enough) square root is 22.3 and some decimals. x2, rounding again, 44-45mm should be fine, could even go down to a 40 with jetting. Will need tuned of course, but will be in the ballpark