Tom at Syc Powersports has a large reliable engine. He'd be the one to ask.
I personally have had great results with my set up. Mine is listed in my sig at the bottom of my posts.
Not sure about being able to double the HP output, but you can definitely make large gains.
As I eluded to in previous posts, the factory mass produced engines are not high quality. They can be MUCH more potent with just some massaging and little $$. There are a lot of casting flaws that can be cleaned up that would allow more air to enter and exit the engine.
Couple that with some head work (porting), camshaft, an unrestrictive air intake and exhaust, and you've got a much more responsive engine. Plus you don't have a lot of $$ into it. If your top end is wore out, you can also consider bolting on a slightly larger 58.5mm cylinder (largest you can reliably go without boring the cases)
Of course you can go big, but you're going to start to get pricey too. You're now also looking to split the cases for a stroker crank, and or machine them for a larger cylinder.
This stuff ^^^ can all be added on later to the original build if planned properly.
Here's some reading info:
http://buggymasters.com/forum/showthread.php?t=4191
Here's a rough idea of what can be achieved:
Post #11
http://buggymasters.com/forum/showthread.php?t=2670