Re: need to engrave hull ID number on older boat
I would simply not do it - there is no way to do so and make it appear original.
It is more likely you will get hit with attempted fraud.
Document the fact that none was engraved by the builder, then add a plate that says something like "for identification purposes this boats hull number is xxxxx" see owner for full documention". The first time you dock in Mexico take that to them and get them to sign off on it.
Lessons learned are opportunities earned.