I think, but not sure but the stems may be different lengths. If you have a complete TX750 front end, forks, trees, etc. Then measure the length of the stem. The XS650 stem is about 8 7/8 inches from the lower bearing seat to the top of the stem.
If the TX750 stem is the same length then you should be able to use the whole front end. If the stem is shorter and you have a set of XS650 trees you can swap the 650 stem into the 750 lower tree.
Leo