If you plan to have an ungoverned engine I would highly recommend using hubs and use kart wheels. The 3 bolt american wheels. Even using high speed bearings in the wheels isnt a good idea. Its not the bearings its the wheels that would be the weak link. I saw a nasty wreck last year from running stock wheels. If you do have your mind made to run the stock wheels at least put some bracing on the inner side of the wheel that faces the mower frame.
You talking about me Willis, I just had to say it. I can tell ya it is not a pretty site to see one lose a wheel and get throwed to the ground and then get run over. It does hurt. Alot