A bad fuel injector will not cause a head gasket to blow. Injectors on Monteros are not known to fail - it's far more likely that you've got fouled plugs that are misfiring and allowing unburned fuel out the exhaust. I wouldn't be running the engine much as unburned fuel will kill your catalytic converter in short order.

Head gasket failures are almost always due to overheating - I'd pressure test the cooling system then plan a thorough overhaul with new thermostat, hoses, radiator cap, etc. Use OEM Mitsubishi thermostat and radiator cap - the Stant garbage at your local parts store is worthless. What kind of radiator did the previous owner install? A cheap plastic eBay radiator is almost as worthless as a Stant thermostat.

Steel Seal is worth trying - beats having to replace the head gasket.