First of all, if it's a 1981, there's no way it's a Ren I or Ren II, those were only made in 1970 and 1971, respectively. After 1972 they were all just called "Renegade" and were no longer a performance package with the special suspension and frame upgrades of the Ren I and II, but were essentially just an appearance package. There NOTHING special about a 1981 Renegade that could not be had on an CJ-5. Just some vinyl decals and a few upgraded appearance options, all of which could be ordered separately. So in short, if you want a Renegade, buy the reproduction decals and slap em on, and no one will ever know the difference. There is no VIN coding or anything else that distinguishes the later Renegade package. Some of them had a clock, but it doesn't appear to be unique to the Renegade, it was part of the Laredo package though.
Yes, the 304 is a Jeep (AMC) engine and was factory available in the CJ-5 in 1981.