These engines are usually available on Ebay, Alliexpress, or Alibaba etc... I believe the 350 is $1100 on ebay & is used in many ATVs. It can probably be purchased for less if going through Alibaba.

Shineray and Zongshen also manufacture 170 or 177MM? 250cc 4 valve race engines that I think are stated near 25hp at $1100. Just thoughts at this time.

I've also wondered an old Honda cb360 might be a good fit.