The lap record for ARCA racers at Knockhill is 59.00 - currently held by a fully modified 75 1.8 turbo though that was some while ago - I'd expect the current cars to be in the 56's. My 33 has 180bhp (1.8 8v), close ratio dog box with LSD, weighs 720kg and runs slicks, my best lap is 60.1. The standard production record stands at 64.54 - thats a 33 with steel panels, full glass, control tyres (Dunlop back then) and a 1700 8v (Class F regs as was - carefully built, balanced, matched, blue printed - but standard cams, rods, pistons, heads and compression ratio within regulation tolerance).
I don't think Tran-X have made any Sud/Sprint/33 diffs for a while now - they are in Coventry if you Google them. Gripper will make one off's I believe, Quaife usually will only do batches. Try Aide at AHM - he may have stock.