Welcome to the site.
Check out my progress.
http://www.buggymasters.com/forum/showthread.php?t=2601
And here are my performance mods in post #11
http://www.buggymasters.com/forum/showthread.php?t=2670
If you wan to do 40 -50 MPH, it's going to cost alot to get there. But not only is the engine work going to be needed, the front ends on these karts are pretty weak. So you wil HAVE to modify it for the additional stresses you're looking to add upon it.
Front end reinforcement is a MUST.
New HD ball joints or heim joint mod will be required