I have done a few Tanakas and KJWs to hpa aswell as a few pistols for the heck of it.
I am a big fan of the palmers pursuit air rigs, and that stems from a long usage with classic guns. Palmers makes a great built to spec rig and for the money is great and has good resale value, a 2nt option is a pre-built Polarstar rig (they offer rigs aswell).
----Tapping the mag---
1. Completely disassemble the mag (The reason for this is to make sure no metal shavings get in the mag)
2. Once you get the entire Mag apart measure a spot in the middle of the bottom metal plate of the Mag
3. Get your fittings (see pic below you will need 1 of each per mag)
-1 1/8th 90 degree
-1 1/8th QD fitting
4. Take a drill bit and make a hole roughly 1/8th of an inch and then get your tap and thread the mag base plate.
5. Teflon tape the fittings threads and then connect them all to each other then screw the fitting in to the mag (see pic below)
6. Enjoy the amazing-ness of your new mod (see pic below, its one of my old Tanakas I had to re-tap a year or so ago)
---Other uses for the mod----
You can tap any gas gun for HPA/Co2. All you need to do is drill the gas inlet valve and add a 1/6th or 1/8th quick disconnect after you re-tap the hole.