Macs makes a nice One Piece Muffler that works on the ST 51. It is easier to get it from Tower or Great Planes/Hobbico. It says for the 40-45 ST but it fits the 51 also.

I used a Tower 46 muffler on mine but it doesn't fit every plane and you have to do a little rattail file work on the exhaust stack to get the muffler to fit. The muffler only costs $15. It'll make your '51 hum.

http://www2.towerhobbies.com/cgi-bin...&I=LXZ653&P=ML for the Macs

http://www2.towerhobbies.com/cgi-bin...&I=LXJB16&P=ML for the Tower

Enjoy,

Jim