As an owner of an Avistar, I think I would have to advise against this. I upgraded mine from the 40LA to the 46FX, and that was more than enough to begin pulling things apart. I had to glass the main wing joint, and where the bolts from the vertical stab go through the horizontal to hold it in place, the holes began to enlarge from the additional stress, allowing the horizontal stab to move around. I wound up having to strip some covering, and epoxying the stabs into place. It's a good thing I pre-flight before every flight, not just at the beginning of the day. Being an ARF, I reinforced the firewall right from the start anyway, but a .61 on here.......not me.

BTW, as a side anecdote, a guy at the field recently attempted the .61 on the .40 high wing trainer idea, stating that he reinforced the firewall. He was flying along, pulled up into a climb, gave it some power....and POP...off came the firewall and engine, still connected by the throttle cable and fuel line. You can guess the result.