My son runs F3, which is on track at the same time as Clubman. His 32 horsepower Moriwaki very seldom gets beaten by a Clubman bike.
if my RS'125R' could run within 5 seconds a lap of a moriwaki i'd consider it 100% success! looks like I have something to shoot for.
I rode my 125 swapped Aprilia 50 at Jennings in reverse back to back with my Aprilia RS250. The info on the bike is on this forum, titled "My Mini". It felt faster through the corners than the 250! But the really tight stuff I think it would lose to the even smaller bikes.
sadly the RS50 stock will get stomped by anything else on the grid... I know this 1st hand. However.... If you gear it right you might be able to hang with the stock xr's. If you put a 125 in it... WOOAH! Sold our cause it was too big for my son. But would have loved to have put a 125 in it.