EZ GO Sport ST II Needs more power?

[lr3]

I picked up a ST Sport II from TSC a few weeks ago to help around the small farm I have in Central Fl. My son and I decided to put it to use and finally get around to filling in the ruts in our road that is approximately 1/4 mile long. I had a pile of dirt fill and loaded it into the bed, we had about 150 - 200 pounds in the back bed plus me 190 and my son 100 pounds. This thing would not budge with that load in it without a push from me, my son had it floored and I had to push to get it moving. Once we got going it was ok but it certainly didnt like it. My son ended up back on his Honda TRX and a trailer so we could fully load the trailer and partially load the EZ GO. I bought it to work but it doesnt like to work at all. The suspension is great when loaded but the engine lacks power. The manual says up to 500lbs in the bed but no way could mine haul that kind of weight unless I was on pavement maybe. I have some ATV trails on my property that we take our four wheelers and horses so I took the ez go on the trails to clear some trees with my chainsaw and the first creek crossing I went through I nearly got stuck. I had to back down the bank and try to get as much speed as possible to make it up the other side. I got stuck on several stumps and my son gets a kick out of watching me walk out of the woods heading towards the barn to get the Honda to pull out the EZ GO.
There is a point to this long story here, basically I need more power; do you suggest shortest belt as some have posted here? If so what size? I would at least expect this thing to haul a bed full of dirt which is why I bought it. Any information from those with experience would be helpful.

Thanks,

Charles