:( ...and we miss you too Jon.
While not a Silvia owner I can guess at what might work...
loctite is your friend when getting a thread to align although you prob should have a scrounge for a MSDS to see which one is "food grade".
Another other choice is to use teflon tape (which you prob should use anyway).
3rd choice could be a spot (just a spot) of super-glue. It will hold it in place, its not a pressure-joint, and you should still be able to "crack" it next time you dismantle the group handle.
The 4th choice is string! Plumbers (at least old timer plumbers) used to use extensive amounts of manila rope to seal threads.