You can get an APL keycap set for your existing Unicomp keyboard, or do a custom order for any of their models. They will even print you custom keycap sets: https://www.pckeyboard.com/page/category/Buttons
The trackball Unicomp is IMO the best keyboard being manufactured (right now they are out of stock until Q3 2021): https://www.pckeyboard.com/page/category/ClassicTrackball
Buckling spring is just a superior (although less durable) key switch technology than the Honeywell switches used in Symbolics keyboards (and most other keyboard switch mechanisms as well). One thing that the Symbolics keyboards did right is having both the APL and Greek legends on keycaps.
But none of this matters if you do not have the right software. GNU Emacs greek and TeX input-methods, and the C-x 8 iso-transl keymap (which you can extend) makes writing mathematical symbols really easy.