I have a friend who is considering a gas engine for her folding Montague bike. She will be climbing mtns in CA, up to 5000 feet gain. (The reason she is considering gas over electric). The most important things to her are it being as quiet as possible, least polluting, and be able to assist her up steep mtn climbs 12% + grades for 10 miles. She ISN'T looking to ride 30+mph. Is there an ideal kit out there that would fit well on the Montague Folding mtn bike? She wants to be able to fold the bike up to store it in a camper. Friction drive would be best right? What 4 strokes engine/kits are there for such an application? Thanks.