You can use any decent mic preamp. If you have an old "tape deck" you know the ones that were a component in 70's and 80's hifi stacks and have 2 VU meters etc. They usually have quite good mic preamps built in and a "Mic" jack on the front and line outs on the back.
Otherwise if your amp has a mic input you can use that.
You certainly can't, mike inputs like that are low impedance - a piezo needs a VERY high impedance, 2 megs is probably a minimum.
Interesting. I have a number of piezo and dynamic mics and never had any troubles using my old Sanyo tape deck mic amp. It must have been designed for both impedance types (ie all mic types).
I've never had a loading problem with any decent mic amp and ceramic pickups.
I am sorry, but is there any other way to design a circuit for this guitar as it has no inbuilt pickups? I already am working on the preamp as Nigel had advised me to.