I know the 250cc Honda's make some good power but I'm not counting on it reving as quick as the 200sx motor, the 250's were designed to be work horses (lot's of torque) where the 200sx was more sporty. Look at this vid:
http://www.youtube.com/watch?v=r9mrdFTfH1g just to give you a clue as to the power of the 200sx in a buggy similar to yours. The 250 would be peppier than the workhorse or Briggs/Kohler. Rapid rpm's is what I'm after in our buggies and the elimination of a CVT, you can't beat a gear driven transmission for raw power when climbing a large hill & reverse doesn't take up anymore space or maintenance,the 300ex would be a choice of engine if it wasn't a manual clutch-it would be a hotrod with lots of top end to spare!! but with kids I prefer the semi-auto's for ease of use and maintenance (lots less fabrication too). The power to wieght ratio of the 200sx over the 250es/ex is huge for the 200sx. You can't get the same thrill from 3600 rpm as with 9,000 rpm-Just my opinion
The 200sx swap was the easiest swap I ever made too! Posts #19 & 20 :
http://www.buggymasters.com/forum/showthread.php?t=1159
What are your exact expectations from the power plant? What type of terrain? Do you just drive or are you into a much more exciting experience?