Hi Nigel, how much engine did you get with your cases? A 5 speed Triumph box is a good choice, Roy Robertson uses one in his Egli racer. Does the Triumph box have a splined or tapered input, clutch, shaft? I recall some having tapers which I am not a fan of, but if you can obtain a shaft which will take a Norton Commando clutch, life becomes a little easier. The Commando clutch, as you can see, uses the Vincent chain size but is 1 tooth bigger bringing it 0.062" closer to the crank. There is quite a bit of enthusiasm on the forum for belt drives & I believe that belts with teeth both sides are now available if you wish to drive a power generation device off the back.
I had a friend who arranged chain case castings which allowed me to cut the diaphragm out with out conscience. However, they were cast using an original & not enough allowance was made for shrinkage, the first one only just cleaned up!
Can you put up pics of your cases, I dont quite understand what you mean by "the bit that some one cut off"? Some have had parts, or all, of the chain casing cut off the side of the crank case, some of those have used modified Comet parts to reclaim them. If you mean the gearbox end of the casing, I may be able to help. I have some new castings which could be welded on, but after considerable deliberation I chose not to take the risk of losing my original engine cases by welding & distorting them. I understand that it has been done successfully? 30 years ago when I started this project new cases were totally out of the question & I couldn't afford, or find, complete used ones! Besides, I now have the satisfaction of reclaiming a couple of sets of original cases and giving them another life. Keep asking questions, I am happy to help. Roy