Just like to know what to change to add a board file to the IDE.
Well, it looks like AT90S2313 is essentially identical to ATtiny2313, so you might get away with just using the existing ATtiny2313 settings. (Check the fuses, though. Or program them from outside the IDE.) If you must, you can modify the "chip" sub-menu of the attinyx313 entry in the ATtinyCore boards.txt:
The AT90S4414 looks like a cut-down and older version of the ATmega8515, so that means a different core, and probably conditionals in the core files themselves to support the differences in UART, Timers, etc. (It looks like they're similar to the existing differences between ATmega8 and ATmega88, so there might be existing conditional code in the relevant places; you'd just have to add the check for AT90S4414.)
See also Platform specification - Arduino CLI
If you've got significant stock of AT90S series parts, you can probably find some commercial manufacturer who "needs" them and will pay more for them than it would cost you to buy a new and better parts. (Hmm. Not counting global chip shortages ) A chip with only 2k of program memory and essentially no peripherals is barely worth using Arduino functions.