RE: Ultra Sport 40 Needs Larger Engine (90?)
Here is a close up of my 60 size Ultrasport with a Jett 90. Well go for it but if your going to use a 91fx your wasting your time. 1. What prop did you use on your 61 fx, 2. Is this the ARF version of the Ultra Sport or the kit? I would give it a shot here but your going to have to do a bit of rebuilding but its worth it to have something different.
You may have to rebuild the cowl area too and make sure that back end is solid and not some framed up surfaces. If you have never see a back end explode, it's a sight to see. Like shooting a pidgeon with a shotgun.
I have done similar, take a perfectly good plane and "bash " it into something useful. I have also dremeled out the bulkhead that allows the fuel tank through but then I run a bead of epoxy around it to compensate for what I took off.