If I was looking for this.....search for the CDH brand.
The CDH 1.5" adapter and 36T gear is $49 I think. That brand is usually pretty good considering....
Not sure about your engine but I wouldn't go lower than 36 tooth on the sprocket.
As IPRocket stated...the CDH adapter halves do not touch. So it can get torqued down tight.
After I bolt mine up tight it never moves. I've read many accounts of people having issues with this.
Clean the hub and adapter surfaces with a degreaser where they touch. I use a little red locktite on that surface too. I guess the red locktite in this spot is a personal preference.
Keep the adapter studs off the spokes when you put it in and lock it down. Tighten the bolts evenly...back and forth in the tightening process. Blue locktite on all the bolts.
Then let it rip...
Good luck with it.
These bikes can test a person's patience. Things seem to never just go in perfectly.... especially in the learning process.