Trailer axles??

[chumbucket]

I too, like most everyone else here, would recommend a tandem axle.
My concern is your statement that the current trailer has 12" wheels. If that's the case, I would seriously look at the trailer ratings if you don't know what it is already. Sounds like even with tandem axles, it's underrated for your boat. Some trailers can be modified and upgraded by installing heavier springs, axles, and tires. Some I wouldn't recommend doing that because of the frame size. The other concern is that if it is underrated and you get in a serious accident while towing, insurance companies may question that fact and or refuse to cover you.
Check that out before you go too far with the rebuild/repairs and decide if it's worth it or not. Maybe a good used trailer is in order if the budget doesn't allow a new one right now. Then sell the one you have to re-coup some of the cash.
Just something else to think about. ;)