I have a broken Digitech RP 150 multi effects pedal, and I need something to control my Amplitube... So I had an idea to make a midi controller out of the Digitech pedal...

I have no Idea how it works, so I am asking you: Can I use the original board somehow, or do I have to buy another circuit board, made for midi control?
Nope. You can't just hack and cannibalize digital stuff like you can analog. It's all ribbon cable and chips and code, and all interfacing, so you can't mix and match pieces and get something functional.