It's certainly possible to substitute silicon for germanium transistors in some designs, but not always a simple swap. You may need to change quite a few other components to set the biasing correctly.
Many older power-output stages using Ge transistors used a couple of diodes to set the base bias. These usually need to be changed to account for the 0.6V forward bias of a Si junction.
From the shorted speaker connections burning out the transistors, I'm assuming the amp has a DC-coupled speaker output -- Quite unusual as most amps of the germanium period would be capacitor coupled (or in earlier models, have an output transformer).
You'd probably find it easier to obtain Ge replacements. The AC187 and AC188 were quite rare (at least on British equipment), but their AC127 and AC128 cousins were very common, and still obtainable, although getting scarcer.
