Quick update- haven't mounted the engine (Honda GXH50) yet (still mocking up all the frame, etc.), but I test ran it over the weekend. Installed the low oil switch, filled with standard 10W30, took out the sparkplug to verify the gap and check the spark, pulled the engine through several revolutions, filled the fuel bowl with a tiny funnel (no tank), started on the first pull. Ran great. Refilled the bowl a few times and ran it up and down the rev range (never all the way up), for several minutes.
So far, so good.
I did not bolt it down to anything and I easily steadied it with one hand throughout the range. I do not have a frame of reference as I've never had a 2 stroke bicycle engine, but it seems to run quite smooth to me. I've seen refereneces here in other post that claimed the engine is very raucous; I'll have to get it mounted, but it seems fine to me. I've worked with numerous larger Briggs and Tecumsehs which run at 1/2 the rpm, and this vibrated no more for sure and feels like much less.