you would need some serious mods to the frame for that! its do able though, i had a 300cc 2 stroke on my go cart i built, scary scary fast, but a 390 4 stroke would need to"spool up" as i say, you could always lighten the flywheel at the cost of a higher idle,
if i were you i would mount something like a 125cc-175cc 2 stroke from those chinese dirt bikes, if you could get the transmition somehow you would fly way faster than with that 390cc there, but whatever you do it will be cool, i just think that that engine cost alot and would be a wast of money,