The '75 CR125M appears to have a small-diameter silencer section with minimal packing. It looks like it would be a real b*tch to repack even after the end cap was cut off; I've never done it and could be wrong, though.
If it were my '75 and I was planning on riding rather than restoring it, and the '75 pipe was in good shape, I would pull that pipe off and replace it with a repackable-silencer pipe. I've got an old LOP 'torque' pipe with minor damage and a new Retro Rocket pipe here, neither of which I'll ever use, and I believe that Jemco and DG both make CR125M pipes; most of these require the larger '76-'78 manifold, though.
I don't know what to tell you about the '78. The 'beer can' slip-on muffler may be a mechanical silencer/spark arrestor unit like the MR250; if it's all clogged up it could be cleaned out with caustic soda or something like that. FMF makes 1.125" and 1.25" universal silencers that work great, SuperTrapp still makes their original line of steel-body disc silencers with a bunch of inlet and core sizes, Jemco makes custom silencers, etc.
Ray