Figured out another bit of the overall design... and jimblumberg, you may be correct on my need to find updated ICs after all - the MC6821 is no problem (W65C21), but I find the MC6840 generates both the output carrier timing AND the ~NMI signal for the CPU.
Well both of these chips can probably be replaced with a simple micro-controller.

As far as a fresh rewrite goes, my plan of attack is to get code for a 4x4 matrix keyboard written
Decoding a 4x4 matrix keyboard is also fairly easy to decode with a simple micro-controller. IMO, it's trickier to properly wire the matrix than decode it.

followed by code to drive an I2C LCD,
Again there are many many simple micro-controllers that have an I2C interface capable of driving a simple LCD. What kind of LCD will you be trying to use? Does it use any kind of graphics or is it a simple single/multiple line display, like a 1 x 16, 2 x 20, etc.

then a proper delay routine
Again there are simple micro-controllers that have built in timing, interrupt modes, and timer subsystems that make delay routines quite simple.

You also need to realize that the hardware you have is all geared to that outdated micro-controller. The 6800 series of controllers were fairly primitive without much of anything available internally. Almost all the functionality is contained in separate chips.

If you go for a complete redesign of the hardware you may be able to reduce the chip count significantly with the side benefit of added simplicity in both chip count and software.