Thank you to all of you for your advices.
Not being a guitar player, the only chance to record midi guitar parts would be: 1) learn to play the guitar (fascinating but time consuming) or 2) use the old balck&white friends (good technique needed for complicated parts). This is why the "tweaking idea" seems so interesting for me.
This 54 years old dummy is a surgeon that about 20 years ago truncated his promising musical career ...
for professional reasons and therefore is now a little bit out of order in the midi knowledge!
I shall try to find the books, the titles sound very promising to me.
All the best
Paolo