It's a 20 MIP coprocessor to make the Tandy faster... just kidding, that's the performance of the bigger Atmels at 16Mhz... it is a small microcontroller, so it's gonna be doing something with timing or twiddling bits for you.
In general, you can replace ATtinys with models of equal or better flash and RAM capacity and if you know for sure you don't need all the storage, of lower flash capacity. If you're having trouble sourcing a 13A in particular.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.