Your Gms .76 matches the The Tower Hobbies .75 exactly. The two engines were made by the same company. I have a Tower .75 with the said pitts muffler sitting on the bench. That I no longer use. I repowered the plane with a .90. Tower used to sell the bison muffler. I will sell you a complete engine gp mount and muffler for the price of the new muffler. The engine is in good shape and runs excellent . It was removed from my shoestring this summer.

http://www3.towerhobbies.com/cgi-bin...?&I=LXGBL0&P=Z