I have had more problems with OHV small air cooled engines ( the newer ones in the last 5 or so years) than any of the older flat head engines by far very very far. If it were me I would look for an older used flat head B&S that in the same shaft configuration (horizontal ? or vertical) any way the same range of horsepower should bolt right up.
Next option look still for a used engine of the same type, I'm not sure if online would be helpful, but you could always check local online etc, and may be able to acquire another unit in which to take an engine from.. I guess next would be to rebuild the engine you have, it may not even need rebuilding, or maybe a cheap rebuild would do it, it's pretty straight forward to rebuild these if they do not need to be bored it doesn't take a lot of money at all from what I have seen however certain engine brands will in fact deem to expensive...
I may have a strange way of looking at this, however I despise the new air cooled engines, they are trash for the most part, and that's not just any brand.. it's across the board brand names pure junk in the entry line.
On edit some OHV engines are what air cooled engines should have been for years such as B&S vanguard which is built overseas BUT in Japan from my understanding is one of the best small air cooled engines ever made, however a new vanguard will set you back a lot of money as well... but it may be possible to find a used vanguard to fit your requirement at a decent price, and it may very well outlast you, and your snow blower.... they are that good if you take half way care of them
