I didn't try your game but here are the specs of the usual engine.
Room speed should be set to 50.
Gravity should be 0.4.
The first jump should should be -8.5 vspeed.
The double jump should be -7 vspeed.
Running speed should be 3.
Maximum falling speed should be 9.
When you release shift and your vspeed is smaller than 0, the vspeed should be multiplied by 0.45 for variable jumping.
Good luck with your engine.