What all was changed when you put it in the quad? Air filters, exhaust, sure the throttle cable is adjusted right?
Might ought to introduce yourself and I'm not sure if you are in the right section. Moderators will be along soon I'm sure to enlighten you.
Not trying to be an ass but might be a good idea to read the rules man.
It's an oil cooled engine...it's in the right section.
So it will rev freely when the clutch is in, and I assume when it's in neutral...but not when in gear.
If it won't pull the load, then you need to look at the carb jetting, float levels etc.
What do you have for a tank and filter? Is it getting enough fuel?
Is it back firing?
Do you know the history of the engine? Did it work ok when it was still in a bike?
Im guessing very long gearing ? Ive seen quad swaps with ridiculous rear sprockets and with the load from the quad ? If the engine reaches the limiter when in neutral easily i see no reason why it cant under load other than bad gearing and too much load