Out of curiosity, what are the biggest engines that you have run on a 40 size Big Stik (or similar)?

I have two Stiks, one with a Rossi 45 that absolutely screams, and one with a rather puny OS .40LA (not knocking the engine) that I want to replace. In the future I want to build a .60 size plane possibly powered by a 4 stroke. Is there an engine out there that would fit in that category but also power the .40 size Stik and handle well in the meantime? Has anyone gone through this same process? What modifications are needed???

By the way, both Stiks are tail-draggers, so big props aren't a problem. Also, the one that I want to switch the engine on is an older Stik that is tail heavy as-is so I think balencing will be a small issue only. I will be recovering this plane also so it is a good opportunity to make modifications if necessary (beefing up the firewall, bigger tank, other strengthening issues etc).