I have friends that have put YS-110's in the H9 .60 sized Ultrastick and the Saito .85 in the World Models P51 but that's the extent of major over powering a model I know of. I like to have a bit more than "stock" engine myself but a 125 Saito in a .40 sized warbird seems like it would give you more problems than fun. It's going to be so nose heavy that the wing loading will be terrible when you get all the tail weight for balance. I had a World Models P51 and I put the OS .50SX in it, I don't think there was much more room than that in the cowl and I chopped a bunch off for the little OS. I also like to run 30% Cool Power Heli fuel in my OS 46FX's. It gives them quite a boost. Not all engines work well with the 30% however, but I think the Saito's like it. Just be sure you get a 30% fuel that has lots of oil in it, around 22% or more. It certainly would be an experiment as far as I know of putting the 125 in a 40 warbird. Sorry I couldn't help much but good luck.