I scratch built a TB-1 from Busco plans (with Palmer's signature) and flew first with a McCoy 35RH and later a McCoy 40RH. It was built light compared to kit built...but I don't remember the weight. The McCoy 35 is 200-300 rmps faster than a Fox 35 and flew it fine with 10/25 Sig castor fuel. The 40 gave much better line tension and is what I settled with (until I crashed it a couple weeks ago). Another flyer at the field is flying his (2) T-Birds with modern OS 35s and they are equal to or more power than the McCoy 40. As much as I like nostalgic engines I would skip the Fox 35 and go 40 on a plane the size of a TB.
The issue is "which 40?". It's not too surprising that a 35FP or the like is stronger than a McCoy 40, and an Aero-Tiger 36 eats the both of them for breakfast. Even a Magnum 32 will out-perform almost all the vintage-type engines.
The difference between Stalker 40 RE (weakest of the bunch as near as I can tell - substantially weaker than a Fox 35) and something like a 40VF or OPS 40 is massive, so we can't just talk about displacement. Really need to specify which engine in particular.