Blowers only work good when the engines are at high RPMs because i used to have a 120 volt computer cooling fan CONSTANTLY ramming air into my minibike's engine carb, but at idle speed, it stalled the engine. but at 3000 RPM and over, it gave the engine at least a 2 hp gain. so then i made it so i had to flip a switch to turn on the fan on when the engine got up to that speed, BUT it was too dangerous to reach behind me to flip the switch while i'm going 30 MPh. SO then i got the idea to hook the fan up to the ignition system on the motor, and it worked PERFECTLY, because now the engine dosent produce enough volts for the fan to spin UNTIL the engine gets up to 3000 RPM. which then when the fan turns on, the engine is at the right speed to take in all the forced air without stalling. AND the engine has not "ignited" on me nor has it even backfired once.
so long story short, if you get the right kind of fan and hook it up to the ignition system on the engine, it will work fine for you.
Another tip for increasing your engine's performance (ONLY ON FLATHEAD ENGINES) is if the valve breather tube isnt already set up to blow into the carb, make it so it does. because one day when i was taking apart my minibike engine (Factory 2.5 HP) and another engine i found (Factory 3 HP) to do the annual internal cleaning, i noticed that the only difference in the carberators of the 2 engines are that the 3HP has the valve breather tube blowing into the Carb, but the 2.5 does not. so i made it so the 2.5 has it blowing in the carb, and when i tested it (prior to cleaning the engine) i noticed a great increase in engine performance.