455 Mower Deck gearbox drain plug stuck

[cef2lion]

I wanted to change my 455 60inch mower deck gearbox fluid. Second owner and I don't think it was ever done. Took gearbox and its mounting plate off deck. There appears to be only
one plug in the gearbox. I don't have a manual for the deck. Assume you take out the plug and tip it to drain it. Problem is the drain/fill plug uses an Allen wrench and it was rounded out.
I tried to use a cold chisel to spin the plug loose. All it did was shear off what was sticking out of the plug. Stopped at that point. Order a new plug and put it back together for now to mow.

Any thoughts on ways to get the old plug out? I'm surprised it was rounded out. It had 200 hours on it when I got it and can't figure pas owner would have tried changing the fluid then.
Wonder if I should take it to a machine shop. Do not like the thoughts of drilling it out with all the metal that would get into the gearbox.

 

[KennedyDiesel]

Snap on makes a spline type screw extractor set. You may be able to drive one of these into the hex without drilling. If not you'll have to drill it I guess.

You simply drive the spline into the fastener, slide the nut on and back it out. If it's really stuck you'll want to use a torch to carefully heat it.