Can the Crosman be converted to bulk fill CO2? That would be straightforward snd give you more shots for less money, filling from a fire extinguisher. Maybe Archers Airguns do one?

PCP conversion sounds like a huge amount of work to do on a couple of low quality rifles. Each to their own but you could buy a pallet of CO2 bulbs for the amount of work and cash you’ll have to inject.

HPA is really dangerous and I find it a bit scary in a purpose-built quality rifle, I wouldn’t go near a $100 throwaway Crosman that’s been pimped to take it. Tubes can explode like grenades or a violent leak can turn the rifle into an unguided rocket projectile..