There are other kits like the rack mounts that can use the Honda or Tanaka engines as well as some frame mount kits that use Honda engines or their clones... It depends on what you want whether it's a small (or large) 4 stroke or the typical CG 2 smoker. There's also the KTM, Morini, and their clones that can be frame mounted, these engines offer the best power for their size, the quality is light years better, but they cost more and you need good welding and fabrication skills to get them installed since there's no kits out there that I know about for the Morini or KTM engines, but if you want a reliable frame mount engine and 9hp or more, this is the way to go.
Personally I'm running a China Girl on mine with a lot of upgrades and port work done to it and it's become very reliable after the engine has broken in and everything's setteld into place. I just do a quick once over about once a week to make sure nothing is come loose, wearing out too quickly, or getting ready to break etc as well as q quick walk around inspection before the first ride each day. That was a different story right after the install because things will work loose and require more attention, but that's typical with any new build...
The biggest maintenance done to mine lately has just been mixing the gas & oil and filling the tank