Larry,
I forget, do you already have a TAV? I looked through the thread, but I did not see any mention. What came with mine is what I would have done had mine come as a "blank slate" like yours. I have the 40 series comet TAV with the drive pulley is on the engine and the driven is mounted to the jackshaft, there is no back plate, the jackshaft has its own frame that is mounted behind the engine on the same plate that the engine bolts to (i.e. this is the frame that is welded on and part of the SB's frame, nothing to do with the backplate on a TAV). Because of that, you don't have to worry about any kind of spacers or anything else for adapting the backplate to the engine. Assuming the drive pulley does not need any kind of spacers (i.e. perhaps the shaft on a Honda Clone engine is longer than the shaft on a vintage Tecumseh and you need a spacer for the driven pulley/clutch work properly) you don't need to make any modifications. However, on my setup, the drive for the rear wheel is on the opposite side as the engine shaft and the separate jackshaft/frame setup does add complexity that a TAV with backplate does not. The advantage to the TAV setup with a backplate is that it is a "compact" and easy transmission setup to retrofit onto these industrial engines.
There's no need for a separate clutch, which I see, at least, the imported ATV's/UTV's use. I believe the TAV type setup that comes with them, they need a separate clutch. The drive pulley acts as the centrifugal clutch with the Comet/GTC TAV systems. Adding another centrifugal clutch would not be, IMHO, a good idea. I don't know of any mechanical type clutch that could be retrofitted very easily to these kinds of engines and I don't think it would be necessary given the internal centrifugal clutch of the TAV drive pulley.
My jackshaft happened to come with a hydraulic disc brake setup, but the "safest" is to use brakes at the wheels because if the chain were to brake, you would have only front brake (assuming you put on a front brake) to try to brake with. Probably not going to be an issue, but it is something to keep in mind. Had mine not already come with the hydraulic setup on the jackshaft, I would have put the brakes on the wheels as on a conventional motorcycle/dirtbike/minibike (I still am planning to put something on the front as my "emergency" brake should the chain break leaving my SB without working brakes). If I were to use the TAV kits with the backplate, the separate brakes on the wheels is what I would use. I do not believe you can retrofit brakes to the backplate TAV jackshaft setups because of the amount of overhang on the jackshaft would put too much pressure on the jackshaft bearings while braking. With the separate frame style like on my SB, the jackshaft bearings are spread far enough apart that the brake is in the middle between the bearings, the driven pulley and sprocket are cantilevered from widely spaced bearings, so the stress is better handled by the bearings. Not sure if the backplate jackshaft has one or two bearings for support, but the driven pulley and sprocket are cantilevered off of closely spaces bearings which adds stress to the bearings and to extend the jackshaft and add a brake would cause even more stress on the jackshaft bearings.
I hope I touched on the majority of your questions.
Paul
