You might even try asking Dave King at Naylor (info@naylorengineering.com) directly of what he may know of being available in the community though not publicly advertised. He’s been having a lot of trouble these past eight months getting his circuit boards sourced from a new vendor, so new Naylors have dried up which might be why the used SD60 market has dried up.
Alternately new Naylors aren’t that much more than used ones, so if his production has kicked back on that might be another avenue and you also get a warranty.
And if you really are in a bind, maybe check out some Buda amps as they’re at somewhat related in DNA though certainly not the same.