First, I would like to emphasize that I have no intention of redistributing. My only purpose is to make a mod 1.11.2 run on a Minecraft version 1.12.2, with the intention of playing it and nothing more.
The mod is called Crossbows Mod and its creator is CameronLow20, here is the link:
You're right. The more I get into the subject, the more I see that it would be very complicated to do this. I even tried to solve problems using Eclipse software but the amount and variety of errors goes beyond my ability to solve them. On the other hand, I've come to the conclusion that it's very interesting to deal with these things, it may become my new passion.
This topic no longer serves me, if it is necessary to close it, no problem.
Thank you for your help.