The Tower Trainer was desiggned with the Tower 40/46 in mind. The fuel line exits the firewall in the middle of the engine mount and routing it to the RNV or standard NV can be a problem. The tower NV is easier to get the fuel line attached than the others. If you choose another engine, orient the RNV so that the fuel nipple points up and not to left (facing the engine). Another annoying quirk it that the tank is so tight that it is necessary to remove the engine and push it back from the front. This has been my experience. The other thing that I might mentioned is that the engine is secured by straps and not actually bolted the the mount. The straps are bolted to the mount. This does allow you to move the engine forward or back a little to help with balancing the plane. Oh, and it will fly well with a 40, with the 46 you will be flying at half throttle most of the time.