Replacing a forklift mast is considered a major modification and under current legislation you would need the OEM's (manufacturer's) approval to do so, also the load capacity plate would have to be changed to suit the replacement mast and can only be issued by the original OEM, with the exception of a qualified engineer overseeing the project and giving his certified approval, on paper of course.
All welding done on a mast or supporting structure must also be certified.