Mitsubishi Sigma 1982 Problems

Phone a Mitsubishi wrecker like Mitspares or Imlachs Auto Parts and they might well be able to supply you with all the parts you need.

YOU could run it on unleaded, but you will eventually wreck the valves through lack of lubrication. The idea then would be to replace the valve seats with the hardened seats such as those used on LPG engines. Other than that, run regular unleaded and an additive such as Valvemaster.