Interesting you have a front fender sitting there. That, along with the blue paint underneath might mean it was a Cat All American version. Still essentially a 300X. Due to the difficulty in locating rear fenders, I'd go ahead and restore it as a 300 without fenders. They make nice little bikes.
Chain, bearings, axles, and tires can be found on the OldMiniBikes Warehouse link above. If you decide to restore it, I have two re-popped foot pegs and one wheel star. Also have a chrome fork spring, maybe a few other things. Half a wheel somewhere.
HPE Muskin put serial numbers on the front, top, left hand side of the engine plates. The first digit, then later the first two digits denoted year, but on some models destined for chain stores, there wasn't always a serial number. Either way, I'd say you're looking at 70 to 74, if it's an All American. I wouldn't say it was a 69, because the 300's of that year tended to have welded foot pegs, not swivel. My two cent's worth. Let me know if I can help.