Sachs made a few models of small rotary engines back during the 1970's. They were used in motorcycles, go-karts, snowmobiles, chainsaws and other stuff. Google "Sachs 303". The old ones trade on eBay fairly often. I don't know if they have made anything newer than that.
I am almost certain
that in the last ten years or so,,,, some company was selling a rotary snowmobile engine. It made about 40 HP, which was comparable to what a typical piston two-stroke in a big snowmobile would.
As for "having one built from scratch", that would be very expensive. And I mean, like,,, ten thousand dollars. Having a machine shop make on-off parts is fabulously expensive. How expensive, you ask?
Recently I had a desire for some custom gears made: these would be made of regular carbon steel, four inches in diameter, a 1.25" keyed bore, 3/4" thick, and needed to have helical-cut teeth. The prices I was quoted for a 30-day delivery, was about $300 each. $600 for a pair of 4-inch-diameter gears. I ended up using steel camshaft gears (which are 2:1 ratio) but only cost about $50 a pair.
If you reeeeaaaalllly insist on doing this, starting with one of the old Sachs is probably the best way--but rotaries are not exactly 'bombproof' in terms of reliability. Read up on the history of Mazda's engines to see plenty of early problems, and note that the Sachs engines never became common even when they were in production.