Re: New ride
If you are going with the larger "small" engines my favorite is the 97cc Lifan flathead. Its an upright engine and is very easy to fit into most common frames. Its also very torquey, smooth and will rev into the upper 4K range reliably. I have run 4 of these in different builds with out issues. They are getting harder to find however but in my opinion worth the search.
I have also run the small HF engines with good results and the price with the usual 20% off coupon is hard to beat!