Depending on your location there may be a lot of M45 cat 4 or cat 4/5 fields in races. M45 cat 4 is often the fastest filling field in NorCal.
If you have your old USCF licence number you may be able to convince the district rep to reinstate you. But as a 3 you'd be racing with the M45 1/2/3 guys and they're really fit. It might be better to downgrade to cat 4, or just start over as cat 5. That's what I did when I came back after 20 years. Since we have a lot of M45 4/5 or 5 fields I didn't race open cat 5 often. The M45 5s are more experienced than open 5s on average and less likely to do something dumb. I mostly do climbing races instead of crits, so I did not find the few inexperienced m45 5s to be a problem as they'd be dropped on the first hill.