IJ, let me ask you a couple of questions. That 900 foot change in elevation is across about 15 miles distance, right? But there are (to the best of my recollection, it's been more than 20 years) a couple of fairly steep grades, one pretty long one, is that correct?
The GEBE should be adequate to the job, if you have one of the more powerful engines, such as the Tanaka 47cc, or the R/S 35cc that they sell. If you have the 25cc R/S I can see where you might encounter some issues. Of course, how much load the engine is pushing is an important factor as well.
Having no knowledge of your mechanical expertise, I'll tell you this - the SkyHawk kits are fine, but they require a good bit more mechanical skill, and more regular preventive maintenance than do the GEBE kits, typically. Other than that, the legal issue is pretty much moot - as the engines SkyHawk sells are identical in outward appearance, so any decal will fit. It's very doubtful anybody is going to disassemble your engine and measure the bore/stroke to see if it is over 50cc.
Welcome to the community, and feel free to chime in with questions, ideas, suggestion, or just commentary. Please use the search function before actually asking technical questions - odds are the question has been asked/answered more than once aleady.
If you'd like to know just what the GEBE 35cc R/S engine is capable of doing, I'd suggest you go read the thread "My Long Ride". Augiedog used one to climb Mt St. Helens in WA state pulling a trailer, and since then has logged one heck of a lot more miles across some pretty bumpy terrain.
