Someone did some work to it for sure.
Here is an R4003. That's the closest Cat I could find that resembled what you have. Note there are several differences, from front end to engine. Rear pegs, no rear fender. I'd get that motor running, and use the same torque converter setup. I cant tell if it's a TC88 series (5/8" belt width) driven or not. I'd swap it out for a Comet 20 system. (This is a separate subject all it's own, but not so hard to do)
In my opinion, after several Cat restorations, (and thousands spend on hard to find parts) I'd make this a rider with the torque converter system and that Briggs flattie. It would make a fine bike, with a lot of hill climbing torque, and a decent top end on the pavement.