I am sure Tony Foale would at the very least be flattered by the request for more frames - let's face it the worst he will say is 'no', and if you don't ask you don't get. If you are married you will know that you can ask all you like, you still don't get, but with frames it could prove different! If Mr F won't restart production, ask him if he would object to any copies being made perhaps. I could (for an exorbitant fee) provide some help, and I have some old cast iron drainpipe going begging too.
Engine-wise he told me the CB900 engine will fit in the same gap - he thought the engine bolts were in the same place too, and suggested I try it to avoid the dry sump dilemma I have had. I opted to stay with the 750 motor because the CB900 is more complicated and a second hand one could be full of fresh problems, I had already got the CB750 engine to start with, and this is such a damn fine forum that I would be drummed out in shame if I abandoned it for a twin cam.
The other side of the coin is that (apparently) Foale frames can handle horsepower up in the region of 140 plus- hence I have loads of scope for tuning as or if funds allow in years to come. Mond you, check with Mr F, don't take my word for it, and everything else would need uprating a bit, but motor options are very wide, and there is so little of the frame that any mounting points could easily be made using some bits of marine ply, some long nails and a very big hammer. Works for me - I ran a Norton Commando for 26 years...