Hy, welcome on Board
Could you please offer us a few pics from your G10??
I have also a Charade G10 from 1981 (5 door)
Here you can look thru all my pics
www.japan-oldies.at/g10
Back to your question.
I would very recomend you to get a CB80 engine from a Charade Gtti. The CB80 engine use the SAME engine block then the CB20 engine you use in your G10.
Best way to realize is to get a complete Charade GTti and build in the CB80 engine incl. wires and CPU.
Georg (JapanTurbo) is corrently stripping one Charade GTti best way if you get in contact with him. But he is also in a other (engish spoken) forum, maybe this is better for you to understand.
But currently I cannot found the URL from this side, but I'm sure he will answer here to you too!
Hope to see you again :)
Greetings
Rainer