I've found the PZ electronics to be fine for what you're doing. 27MHZ systems aren't the greatest, but the setup made for the PZ planes is pretty good for for a park flyer. I'm using one in my Slow Stick.

you didn't give the demintions on the plane you're building, but it at a guess I'd say its too big for the Piper's 370 motor and 5:1 gearing. The F27b uses a 480 motor, which would be fine, however it's also a very light plane, so a gear box and new prop will be needed.

Your best option would be a P51's gearbox, however the motor would need a pinion gear, and the f27's shaft is splined, so your best and easiest bet is to just buy a p51 motor and gearbox, and the P51's prop. Or for that matter, any 400 size motor and 3:1 gearbox should do, the PZ's would just be the easiest to wire.