I think multigrade oils are better for every engine. But, if the motorcycle is air cooled (no water cooled motors, they *need* multigrade) and the manufacturer recommended a single-weight oil when the motor was built, it won't harm anything.
I used single-weight 50 for quite a few years before switching to synthetic 20W-50, but that was because I had two 20-litre pails of AeroShell 100 (aviation 100 = SAE 50) to go through, that I got for free when the company I worked for converted the engines in the two DeHavilland Otter w/air cooled radial engines that they (still) own, to turboprops.*
You can't resell aviation oil, for the most part, unless it's in fresh, unopened containers (which these weren't), and the turbos don't use it, so a nice gift to me.
But my motor is 60 years old (built in Milwaukee in November 1957) and called for 50-grade when new. I think the bigger question would be can you run multigrade oil in an air-cooled four-stroke motorcycle, and I believe the answer is yes, and further if you can, you should.
Now that I look at my post, I should change my sig for the big twin oil.
* Powerplant: 1 × Pratt & Whitney R-1340-S1H1-G Wasp 9-cylinder air-cooled radial engine, 600 hp (448 kW)
Turbo conversion: 1 x Pratt & Whitney Canada PT6A-34, 750 shaft HP