Cutting the cowl for a different muffler placement might not be desirable, but it's not that big a deal to re-position a throttle linkage or even the servo for that matter if you decide to change engines. But I have to agree with Warbird40 that a Magnum .70 fs would be a fine choice. I have a .91 Magnum fs in a 110" Cub and I am well pleased with it, but I think a .91 would be a little more than what's needed for that 81" GP cub. Just my opinion of course.