If you want to go custom frame use a pit bike engine, heres a build thread using one.
https://motorbicycling.com/threads/latest-lunacy-honda-ct70-clone-build.36108/
Its steel but its a 29er cross country bike with a shift kit
I've got a shift kit lying around here and half a mind to use a torque sensing bottom bracket with it to have a gas build that can compete with a toseven dm01/cyc photon. but it would be a real pig unless I was counting grams on every component. And if I were counting grams I would go friction drive and a mechanical throttle.
As for aluminum mountain bikes, lowracer has done a couple:
Thread 'Pocket Bike Engine Project 3 (V Belt)'
https://motoredbikes.com/threads/pocket-bike-engine-project-3-v-belt.35861/
Thread 'Tanaka PF-3300 V-Belt Drive w/Power Pipe'
https://motoredbikes.com/threads/tanaka-pf-3300-v-belt-drive-w-power-pipe.36996/
I wont say vibration concerns are overblown, just that failure wont be instantaneous but should still be non destructively tested every so often. And that there are better options for weight savings or more up to date geometries.
Could always take a cheap steel frame and braze or weld a curving top tube on it.