A correction to an earlier statement here: the FAA does NOT require deactivation of an ejection seat, but it's a pricey annual inspection. Look for ads stating "Hot" seat in surplus military jets. I wouldn't want to fly a MiG 21 without one. The pilot couldn't get out at all.
All that stands between most pilots and ownership of a MiG is the fuel cost at ~6.00 / gallon for 700 gallons in a MiG 21. There are several hundred Czech built L-39 Albatross fighter/trainers flying here in the U.S., and they are very popular. Flight training in one costs $1,500/hour, and 10 hours are required for qualification/logbook endorsement. An FAA checkride finishes the deal with a fighter jet.