FiggyG---don't worry about the weight----your UCD will fly great. If anything, your engine is somwhat big----I'm flying my UCD with a Saito 100 and it's plenty of power. However, sounds like your engine mounting dimensions are about right-----go ahead mount your cowl and fly. I've found the cowling can be moved a great deal either fore or aft to accomodate just about any engine mounting. I'd definately recommend ditching the control rods supplied with the kit and replacing with carbon fiber rods. Much stiffer----which you'll need with a 125 engine. The UCD is very light for its size and I'll guarantee that you'll be pleased---in fact, I doubt you'll be a full throttle very much. The great fun of this airframe is its SLOW speed characteristics. I typically fly my UCD (Saito 100) at half throttle)---and it's fully aerobatic at that setting.

If your engine is mounted inverted, you'll want to consider a Cline regulator. I found that inverted installations in this plane tend to result in the fuel tank siphoning (fuel tank is higher than the carb) when you fill it--and the carb tends to flood. Also, with the engine mounted inverted, I found the fuel flow to the carb can be uneven (sometimes too rich, sometimes too lean). A Cline regulator will eliminate this entirely---they cost about $60 and are worth every penny. I've had one on mhy UCD for 2 years and it works flawlessly. You also might want to consider a remote glow ignitor located somewhere conveniently on the fuse or cowl----with the engine inverted, attaching a glow driver to the glow plug can be challenging. I didn't put one on my UCD and I now wish I had.

Chris8105, I think your Magnum 91FS will be fine. I flew a UCD with an OS91FS for a couple of years and liked it. Again, I think you'll be flying your plane at half throttle most of the time---slow speed maneouvers are the real fun of this plane. 3D hovering might be marginal with the 91FS----I don't hover, so I can't comment. Again, I'd recommend a Cline regulaotr or Perry pump to solve any fuel siphoning or fuel flow problems. Get 'em assembled and fly 'em guys. For the money, I think the UCD is the hands down best buy and most fun you can have in RC......WarpedWing