New member and think about a project

If yer interested in both the SBP shiftkit and engine modifications, then odds are you'll be getting one of the two stroke kits... fun & inexpensive, there's a plethora of mods and aftermarket parts for 'em, yet I've a frame roughly similar to the 2nd one you've shown and I wanted ta give ya a quick heads up - not sayin' it's impossible ofc, but the way the top tube and the down tube are joined in the front & they way the top tube "drops" as it gets closer to the seat post, while it's really strong, it changes the angles within the frame a bit and reduces the amount of room... the problem being if you can indeed fit the engine (and ya prolly can), the angles of the motor mounts may not be the same as the tubes, resulting in a poor fit with the edge of the motor mount "biting" the frame.

Havin' said that, my frame had a bit more of an extreme angle problem than that so you may find it isn't a problem - but if ya do, the rear motor mount spacer block can be modified, it's aluminum and can be easily filed/sanded to change the angle. There's also U bolt front mounts available & those aren't angle dependent (well, sorta but again easy to mod), but they take up a lil more room... and worse case scenario, it'd fit in the above Trek like it was made for it lol

I don't think it's an insurmountable problem, but I thought I outa mention it, welcome to the forum lol

Unsure if a frame-mounted Happy Time engine will fit in there. There are measuring specs on this forum or the other forum. Measure before buying.
You could always run friction drive, GEBE or Staton chain drive on engines from 1.5hp to 4.2hp+. I'm building Project 29er. I have a steel 18" Kona frame and an aluminum 18" Diamondback frame. I'll be using frame-mounted Tanaka 47R 2.8hp engine or 4.2hp GP460 engines. They should be fast and smooth.

I'll be watching your build with interest.