Harbor Freight has an 11 HP w/ 1" shaft for $299, you can look online and print off a coupon good for 20% any one item, gets it down to $240.
And from what I've seen at the stores, they usually have them in stock and ready to go. If you go that route, I'd not get the extra warranty, if it lives through the warranty, it'll go on for years after that
While I try to buy American as much as possible and stay away from Chinese as much as possible, those kind of prices make it hard not to get one. My grandsons go-cart blew up and I couldn't find a Briggs, Tecumseh or small Kohler anywhere in stock. Got a 6.5 HP from Harbor Freight and thing has run perfectly for two years. They all have a fuel shutoff lever on them so you can run all the gas out of the carb when you're done, they've got low oil sensors, etc.
I have read that you need to flush the crankcase out before you use it just in case there's metal shavings left inside, but I read that after the grandsons had been running for a year.