Model 50 silencer

Are available to insert into either end of an exhaust can. Don't know which silencer you are running but there are companies who will re stuff your silencer. Just search on the web.
 

I've got a pear shaped silencer on my Model 50 and it's also completely empty. The sound is also a bit too much to feel comfortable with, and your post has reminded me to do something about it. Which I always think when I ride, but forget as soon as I get home.

The first option that I've thought of, is rolling a perforated metal sheet into a tube, insert it from the rear end (don't even think about making a joke...) and fixate it with a parker screw. However, I haven't thought about how to get the stuffing around it (basalt wool, as other stuff seems to melt or blow out). Maybe it could be as simple as to just stuff the silencer, push the tube in and remove anything inside the tube.

A straight through perforated tube like that will absorb the high frequencies, leaving just the low ones. Baffles will do the opposite.

The other option is using a VW Beetle dB-killer that can also be slid into the exhaust and fixated with a parker screw. However, I don't know how the extra pressure of that one affects the running.

Peter's comment above is sensible.  It's what AMC did on the AJS/Matchless singles, but to my knowledge the downpipe was blocked at the end so that all gases had to go through the perforation. It certainly took the 'sting' out of the sound on my dad's bike.