It would be possible, sure, but difficult. Have you ever looked inside one of the old machines? Mechanically complicated. I'd compare it to building your own mechanical alarm clock. This is why the "genius" moniker is often given to Charles Fey.

Older slot machines in crappy condition are available on and elsewhere relatively cheap. Restoration of an older machine is fairly easy. Reproduction (in some cases NOS) reel strips & decals are also available.