thanks for the input mate!
I just did some more research. I found they do make a 2 into 1 carb manifold to run a mikuni bst 40 carb on the earlier xt660 engine. these are on the grizzly 660 quad bikes. I'm hoping the flow meter is the same diameter and can be fitted to the standard Carb head without too much modifying using the grizzly manifold.

my dash is damaged on the szr so swapping the complete loom/dash and efi would be a way around it.

I think the pickup I mentioned earlier could be a speedo sensor... this may be a problem as the earlier engine wouldnt have the speedo sensor that bolts into the casing... I would have to find a way around it...

maybe just sell the carb 660 engine and put some additional cash into getting an efi 660 engine.

I'll compare the frame mounts between the xt660 and the szr660 tomorrow to see how close they are....

any input Kev?