Assuming that you have checked for a restriction in the gas line, a spider web in the orifice or venturi, you might try Landman USA, they took over GOSM, I also saw somewhere that a fellow Q'er that lived not far from an old GOSM plant had some burners to sell, might try a search for GOSM burners, ...when I bought my GOSM from the ACE Hardware I asked about replacement parts, they said it wouldn't be a problem, they order from Ace Hardware in Carson CA (310) 603 2800, might try them.
Hope this helps,
Gene