Id like to know how you plan to do change the timing also.
I have heard they make crankshaft extension (thingies) that are keyed differently for different timings. I here that Go-karts racers use them but most of them just advance timing. i guess you could machine one.
To answer your question. Yes, you should not have any problem running your 5hp on 15-25 LPM of HHO.
You can probably run it on a good bit less if you go direct to the intake and restrict fresh air. If you want try that you need to take some extra precautions.
When you're one step ahead of the crowd you're a genius.
When you're two steps ahead, you're a crackpot."