i have been looking for a 2 into 1 exhaust header for my bike as mine is starting to crack and rust away.
do any other models fit the 750 like the 906,907i or other model 750?

All models mufflers fits but headers don’t,
750 smaller diameter.

Also the 750 cylinders are not as tall as the 906 or 907 so the heads are closer and the headers are not going to fit. A slightly larger header dia ( like 3mm -6mm larger) is cheap tuning . If you have the old headers as a pattern an exhaust shop may be able to bend up a new set .....maybe even in stainless. I have found that when headers start to crack no amount of welding up the existing cracks will stop the ones you cant yet see from developing further.
I made a header for my old suzuki quad and it was obvious that it wasnt quite right as there was a bit of stress getting it all bolted up / mounted , so I heat up the header at major bends ( u bend at ex port and kink around frame ) while still mounted untill just about melting and when it cooled it was a much better fit. I think the pipe sorta did its own stress relieving /alligning thing whilst nearly liquid.

thanks for the info, i will look into getting as set of headers made.