Has anyone ever done this?

Interesting information about Gussets and Brazing. Thanks.

Overall, if the frame is steel with quality welds, I’m good! I want to focus more on heavy duty wheel bearings and good wheels to handle the speed. At 100mph, those bearings have got to get pretty hot!
I really do think you need to dial back the expectations. For a homemade machine 100kph with all the gear is tempting fate, let alone 100mph.

A kx65 engine would work well. Those take 420 chain making the sprocket situation easier.
If you could get a stout usa made schwinn beach cruiser, those will take k10 forks without too much trouble. Just a matter of filing the rear forkends to fit a 12mm axle if you want a matching wheel rear wheel. Check out treatland's wheel hubs. Can get a rear drum brake hub with sealed bearings, and a sprocket mount for not too much.
I was thinking of one of those CT125 China clone motors, like on a Honda cub. Won't be going 100mph. You'd have a fun ride and way simpler to get it done.
Thank you guys for your thoughts and ideas! If I actually get this thing together, I’ll post pics and videos.