Welcome! I say read around here, shop some on eBay, there are a lot of similarities and just a few differences, so I won't recommend one supplier. To some extent - there are so many similarities, it's more what you do with the kit than the single guy you buy the China kit from.
welcome gusjdj. i would really like to see how you modify a mower engine to the bicycle. i imagine it will have great power. as far as the kits go i have only bought one so i cannot really recommend one over another. all i know is they are a lot of fun and fairly easy to put on a bicycle with a little modification. glad to have you with us