If it were me, I'd opt to remove the upgraded subplatter, glass platter, and upgraded cartridge and sell the P1.
You could sell the P1 with the glass platter included if you like but you can install the upgrades on your newer Rega deck, be it a P3/24 or P5 if that is the route you choose.
It's not worth upgrading the tonearm or motor on the P1. The 24V motor will be at least $189.00 and the RB-301 will be about $495 so you can see that unless you buy used, you'll end up spending more money than necessary. You'll also spend a few hours installing both. Just my opinion of course.