As the injectors will be operating under different pressures and with different fluids they will open/close at different rates so each will need its own control circuit. For a 2-stroke system at 5000 rpm the pulse repetition period is 12mS. Simulation shows that for a 2mS pulse duration the average dissipated power when driving a 2 Ohm 2mH injector is:-
Injector 1.4W, FET 4.0W, Spike suppression zener 3.0W, Current sense resistor 40mW.
So a 0.5W rated sense resistor should be adequate.
Be aware that during the hold part of the cycle the FET tends to hunt about the hold level, i.e. it oscillates, so will generate considerable RFI which will need to be suppressed. A darlington instead of the FET would be less likely to oscillate (lower parasitic capacitance).