Absolutely salvageable without putting too much into it unless something major like the power transformer is blown. At the very least, you'll have a project to poke around with. There's a demand for those Sivertone amps. This one is a model 1465, rated at 150W, built by Danelectro for Sears. The schematic is below. It came with different speaker configurations, one was a 610. It has a James tone stage, similar to the one used in Ampeg amps. The power supply caps might need changing.
The best advice I can give you is take your time and do no harm. Remove the amp from the chassis and start with a good cleaning with a damp cloth. Be careful if you are removing the rust. The best apprioach with the transformers is to lightly brush off any loose particles and stabilize the surface with a rust paint. Do not sand the edges where the transformer plates are stacked, the metal frames are fine to sand if you wish to. They should be painted for rust protection. Be careful with the lettering on the faceplate and the chassis. It might come off in a heartbeat if you use alcohol to clean. Dab with a wet cotton makeup removal pad, don't scrape. Move up to a mild soap and water solution if necessary. More agressive foaming type bathroom cleaners (I use a product called Scrubbing Bubbles) can be used on chassis bare metal.
There are a few helpful tips in the TB Portaflex Wiki:
Preventative Maintenance
Cleaning and Lubricating a Potentiometer: How and Where to Apply Deoxit
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