Kind of a cockup set of engine rules. With the 74 cut-off for P1 the two main engine choices would be BMW & Yamaha XS (with 750 kit). As those are the same dominate engines choices for their P2, kind of kills off P1, given similar wheel & tire rules.
My .02, a better version would have been:
P1 - 500cc 4 strokes, single or twin, sitter, or kneeler frame, 16" wheel, 67 cut-off
P2 - 500 2 stroke twins (cut-off??)/ 750 4 stroke twins (max 8 valves, allows Weslake / Nourish head Triumph), 15" wheel min. / 5" tread, 74 cut-off (allows electric start XS 447 cases)
P3 - 4 stroke 750, max 4 cylinder, minimum 10" wheel / treaded tire (max 6" tread??), 74 cut-off (allows Trident 3, and Honda SOHC, and period frame designs like 10" Windle)
P4 - 4 stroke 1000cc, any cylinder configuration, minimum 13" wheel / slicks (max 8" tread??), 82 cut-off (allows Z1, CB900F, CBX, GPZ, others??).
"Anyone can build a race car, the trick is to make it fast." - Harvey Postlethwaite