I do have a MX067 pwm modified to use a P-channel mosfet that is working.
Basically the P-channel fet switches the positive voltage from the battery. I've only found one mosfet that is a good substitute for the N-channel fet we have been using.
The problem was finding a P-channel fet with the same Vgs(th) gate threshold signal as the N-channel fets use that we find in our pwm's. Part # STP80PF55 has the same gate threshold signal requirements and can be used in our pwm's. The gate ( pin 1 ) and drain ( pin 2) connect the same way as the N-channel fets. The source ( pin 3 ) of the P-channel will connect to the positive battery post and of course ther N-channel source ( pin 3 ) connects to the negative post ( ground ).
How I modified the MX067 ( MX068 uses same schematic ) follows:
1) I cut the ground run from the driver portion of the pwm to where the ground spade terminal is.
2) I cut the run between L+ and positive spade terminals.
3) I attaached a wire from L+ spade to the ground run returning to the driver circuit.
4) I removed the flyback diode - D1.
5) I removed the N-channel mosfets and installed in thier place a P-channel mosfet.
How I connected the pwm to the HHO electrolyser:
1) I connected a positive voltage from the ignition circuit to the + spade terminal on the pwm.
2) I connected ground to the L+ spade terminal on the pwm.
3) L- still goes to the HHO electrolyser.
4) The ground spade connector now connects to the positive post of the battery ( or the HHO relay )
I do not know if this will work better than N-chanel pwm's. I believe it will work better. When I was testing it my resovior and electrolyser were leaking. I'm in the process of making repaires. One thing that I did notice is sthat the higher PRF I dialed in the current would drop. This is with a 60% duty cycle.
I should know more this weekend.