It's by no means unfeasible or illegal,just get the truck certificated by some-one qualified afterwards.....I'd suggest me but I'm too far away!! lol

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.

This is not rocket science. With a good welder and the proper size mast, width and capacity, it can be retrofitted. Let's face it, this is a cheap lift they are talking about and not some heavy industrial application.
We have done it for years. If you don't know what you are doing though, you would be well advised to walk away from it.
The liabilities are too much and the possible gain is minimal to begin with.

YOU MAY FIND A MAST AT ATLANTA FORKLIFT SALVAGE.
THEY HAVE AN AMAZING NUMBER OF RETIRED FORKLIFTS AT THEIR FACILITY.
WE RECENTLY FOUND A MAST FOR A CLARK TWO MODELS OLDER THAN YOUR CY50B MODEL.
TELL THEM WHAT MODEL TRUCK YOU HAVE AND WHAT YOU ARE LOOKING FOR.
I DON'T HAVE THEIR PHONE NUMBER BUT I SUSPECT YOU CAN FIND IT ON THE INTERNET.
NOT ALL MASTS WILL FIT YOUR CHASSIS.
GOOD LUCK, GLEN