how bad is this flash ?
I've got a few ancient 283 and 285 units that display some some of that same "cooking" on the front diffuser, though I've never seen it this severe. I think it is O.K. to go ahead and buy the unit so long as it is cheap and works during a test fire. I wouldn't mess around with a "fix." Liable to do more damage than good, especially since your stated needs are non-critical.