If you had a bike with smaller frame tubes you could use PVC 1& 1/8" black irrigation tubing, like I did on my 1950's J.C. Higgins. I would not go with any softer shim material.
The big problem with rubber mounting the engine is most if not all of these engines come with eccentric engine driven chain sprockets, so at 6000 rpm you have the output sprocket turning 1350 rpm and it's tightening and loosening the chain repeatedly.
That cyclic chain snatch will break stuff, it will snap rear wheel spokes, bend and break engine mount studs, strip out the mount stud casting's threads, strip the exhaust stud casting's threads in the cylinder, (if the muffler isn't braced firmly to the engine...) it will rattle parts loose on your bike, throw your float adjustment off with the thin float tangs in the NT carbs. You get the picture.
If this is your first motorized bike conversion, it's always best to choose an older "V" framed MTB or old British or US made 3 speed frame. That's exactly the bike frame that these engines were built for. The solidity of the engine mounts are a big deal, and should be tight and not wiggle at all. Everyone likes the cheap, curved downtube style cruisers for motorized bikes, but there no worse bike you can choose for this use.