Re: What would you do different?
From my first build experience...
Start with a quality engine like Dax or better
Start with a frame that'll easily fit the engine
Have all the parts necessary to complete the build before starting (I'm still guilty there sometimes)
Although my first build was very successful I did have my share of challenges to overcome like starting out with a bike that had just barely enough frame space to fit the engine, this lead to some serious modifications to the frame and mounts to get it to sit in the frame rigidly. I bought an engine kit off ebay thinking they were all the same, this engine lasted me about 90 minutes of run time before I found a critical problem with the crank. By this time I found this forum and was reading up on everything, and it was then when I found out some of these engines are a lot better than others so I completely ditched my efforts to repair my first engine and bought a Dax bottom end to put my top end on, I also ended up buying a new carb since the one that came in the kit kept shedding off rubber from it's built in fuel valve causing the engine to cut out randomly, I did fix the problem but decided to use the better carb.
My rear mounting setup was rock solid but my front mounting system used a steel bar going thru a slot I cut thru the frame for the engine to bolt directly onto. Thinking these engines couldn't make enough power to cause problems I rode the bike like this during testing and while awaiting enough free time to paint, I was going to weld the steel bar solidly into place but those few hours of run time kinda added up and I found a nice sized crack in the frame caused by the slot I cut for the mount to work... Lesson there is to go ahead and weld everything into place BEFORE riding.
Best thing I can tell new builders is to use the best quality engine and kit to start out with, and TAKE YOUR TIME with your build to get everything right, don't assume a frame is strong enough to ride on until everything is welded up and completed. These engines aren't very strong right out of the box, but can get that way real easy once a little porting is done and a good pipe is added...