Possible to repair Hunter PGP heads?

I have a sprinkler system with 4 zones and a mix of mainly Hunter PGP heads (90%) and some Toro heads that have been running forever according to my neighbor who was around when the system got installed 20+ years ago. The controller is a Rainbird 7i also from 20+ years ago. All mechanical parts. It's a bit noisy but it's supposed to be a workhorse, though I had to replace the motors in it this season.
In fact, this season has been a nightmare. The controller died, I mangled a line while pulling out a tree root, station 1 will not operate (I suspect the solenoid went but don't have a fault finder to find the underground connection) and the service can't come out until the end of the month and several heads were non-functioning this year.
So, I had to realize that every season I'm replacing 1 to 3 heads without fail. They're not expensive but for 90% of them, they just stop turning. That got me to thinking that perhaps there's a more economical way to deal with this. Can these heads easily be repaired to go back to proper rotation? In most, if I turn them back to one side, they'll turn, return and then stop. Some won't rotate at all any longer. Or is my only option to just keep buying new ones?
In the repair I made for the broken PVC line, the Hunter head was working just fine until I repaired the line and re-aligned it. Upon screwing it back in, it was completely out of alignment. I did this with the water on which I've now read is a possible cause, though I've seen the irrigation guys do this all the time.
Any help would be greatly appreciated.
*** RTH ***
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