welding aluminum steerer stem

Does anyone in the framebuilders forum, with extensive experience welding aluminum, offer a repair service? I have a handlebar/steerer stem about a foot long (for a folding bike) that had an integrated (welded) handlebar clamp that cracked. I would like the handlebar clamp sawn off and then a smaller aluminum tube inserted telescopically into the tube maybe four inches or so, protruding from the top about two inches, and welded in place there, so that a standard handlebar stem could be bolted onto it. The proprietary replacement from the manufacturer is just too costly.

I've included a picture of the stem with a pencil mark in the approximate location where I'd like the outer tube to be cut.

Is that kind of repair job even feasible?