Hello. This is not an infrequent question, whether from fears of a great pending catastrophy or breakdown of civilization to anti-freedom, anti-Second Amendment types making it harder and harder to do anything that is firearm-related, including send handguns for repair. Some want to know just because they like having the parts for a near immediate fix.
For over three decades I've been shooting 9mm Hi Powers quite a bit. Parts breakage has been limited but can occur to be sure. Based on what I've seen break, here is what I suggest having spares of the following:
Firing Pin Retaining Plates: (If these crack, it is usually at the 7 o' clock position when looking at the plate from the rear.)
Extractors and extractor springs: (The Hi Power requires a strong extractor spring. Usually when I've seen one with extraction problems, simply replacing the spring has solved the problem. I prefer the extra-strength extractor springs from Wolff.)
Firing Pins: (I've not had these break but do have a couple on hand should that occur.)
Recoil Springs: Changing these out is essential in Hi Powers that are shot very much. I change mine about every 2K to 2500 shots. The Hi Powers I've seen that had damaged, rounded lugs always had extremely weak recoil springs.
Ejector: (I have not seen any of these break, but if one should, it would be nice to have a spare.)
Grip Screws: These can be lost or damaged.
Spare Roll Pins: for extractor and sear lever (If you have a Hi Power having the factory extended ambidextrous thumb safety, you might want a spare roll pin or two for it, too.)
In my opinion, one cannot have too many magazines and springs as well. I try and have plenty for any semiautomatic I own. I consider 5 or 6 the barest of minimums. Having said that, I suggest strongly obtaining only quality magazines. My first choice remains Mec-Gar 13-shot 9mm magazines, be they marked "Browning" or their real maker's name.
Though breakage has been pretty darned rare, these are the parts I'd have on hand. Once these are acquired, it doesn't hurt a thing to pick up other parts just in case of the very rare possibility that something not stressed gives up the ghost, so to speak.