It won't work ?, well, not the way you hope.
If you can bend stuff so none of the welds are critical for strength, maybe, but typically plastic doesn't 'weld' like metal. The joins won't be as strong as the bulk material. Not saying plastic is bad, some of the moulded plastic guards are excellent, but they aren't made with welds.
I've certainly fixed things like indicator housings and handguards, but 'structural' strength is going to be a hard ask.