Unlike some machines, the 28 keyboard is not a self-contained add-on assembly.
Someone had asked earlier about converting a KSR to an RO, and after looking at the two in my basement I concluded it was possible, so “probably” the reverse conversion is theoretically possible. You would have to source a whole lot of pieces though. Maybe several hundreds of individual parts…
Keyboard frame, keys, keylevers, code bars, transmitter, camshaft, driveshaft, etc etc etc. maybe have to drill some more holes in the RO base.
Except (with Teletype there is always an Except), the 28 compact does have a M32 style electrical keyboard assembly with parallel output so you’d have to add a distributor as in the 28 compact KSR. I can’t figure how you’d do that on a conventional 28 base.
Far far better if you can find a 28KSR base/keyboard. Rara Avis in Oz I imagine.