Engines are cranky in MSFS
If systems.cfg file and engines.cfg file don't agree in some way, you're not starting anything. Check your fuel also, some aircraft load up with empty tanks. As a last resort, slew up to altitude, that normally fires up engines if they're able to work.
Thanks for the suggestions DC but that wasn't it, plenty of fuel in the tank and airstarting it didn't work.
I have however fixed it
I had been testing the N2S/PT-17 variant and hadn't yet tried the Model 75 so I did and it fired up just fine, so now I had something to check against.
First I compared the engine and systems as per your suggestion, Systems where the same but the engines where different, they do have different engines though so that wasn't a total surprise.
However the N2S lists the max_design_mp and min_design_mp as 0 which can't be right. I tried both copying the numbers from the Model 75 over and copying the whole engine.cfg over but still no go so that wasn't it.
Those numbers can't be right set at 0 in the N2S though, anyone know the correct numbers and would it make a difference how it performs ?
Then out of curiosity I compared the flight_model.cfg, although I didn't think there would be anything in it that would stop the engine from working, I was wrong. There's a [FUEL] section and in there is
electric_pump = 0 ;
engine_driven_pump = 1 ;
manual_pump = 0 ;
In the N2S they were all set to 0 so I set the engine_driven_pump to 1 and it fired right up.
Success
now I just have to test fly them.
A couple of the gauges dont seem to work correctly, the fuel gauge sits at just over zero and the engine temp gauge is stuck at 40, they maybe fixable in MCX so I'll look into that later.