Hmike's right by the looks of it. If the engine hasn't been swapped already, it's an ST295. Any of the later frames should fit in the body, and I'm sure one of us has a useable unit already out. The '73 and newer frames use a different chain routing, but it doesn't create a problem.
Congratulations on the Attex. They're a great machine to have as a first rebuild: simple design, parts easy to find or fab, and lots of fanatics out there that have done it before.
Good Luck!
Stuck in the seventies- not in the swamp.
(6) Attex, a Hustler, a Super Swamp Fox, (2) Tricarts, (3) Tri-sports, a Sno-co trike, 3 Dunecycles, and a Starcraft! ...so far