You will need to do the following:
Fit an 81 Type alternator stator casing to get the pickup coils and 81 Type rotor to get the pickup magnet (although you could drill an older rotor accurately and fit a magnet).
Modify the brushes on the 81 alternator stator casing to match the 75's Type B regulator.
Fit a dual spark ignition coil for the TCI to drive.
Add extra wiring to connect the the 75's harness and coil into the TCI.
These are not big issues to address, but is it worth the trouble?? I would personally find it much better value and easier to fit a Boyer Bransden electronic ignition system which is modern and reliable. Failing that I would retain the points ignition as long as it was all mechanically sound, particularly the advance mechanism.
Have fun and enjoy the XS.