Mixing NMOS and CMOS
You can replace your HN27256G-25 (250 ns) NMOS EPROM with for instance a M27C256B-10F1 (100 ns) CMOS EPROM. They even have, in stock.

Most important things to check:

They are pin-to-pin compatible.
Supply(CMOS)=5 V = Supply(NMOS)=5 V.
Access time(CMOS)=100 ns \$\le\$ Access time(NMOS)=250 ns.
Operating current(CMOS)=30 mA @ 5 MHz \$\le\$ Operating current(NMOS)=100 mA max (!).
Standby current(CMOS)=100 \$\mu\$A \$\le\$ Standby current(NMOS)=40 mA (!).
\$V_{OLmax}(NMOS)=0.45\;V \le V_{ILmax}(CMOS)=0.8\;V\$.
\$V_{OHmin}(NMOS)=2.4\;V \ge V_{IHmin}(CMOS)=2\;V\$.
\$V_{OLmax}(CMOS)=0.4\;V \le V_{ILmax}(NMOS)=0.8\;V\$.
\$V_{OHmin}(CMOS)=3.6\;V \ge V_{IHmin}(NMOS)=2\;V\$.  

All that is fine, so the CMOS EPROM should work.

Russel is right in that there are several other things to check, related to timings but, being the CMOS a 2.5x faster device, odds are that the new device will work with no problems. And the reason for this is that memory interfacing protocols are designed precisely with that in mind: so that devices with shorter delays will not pose problems. Manufacturers specify maximum delays, not minimum delays. They also specify minimum setup times, but that's another thing, which doesn't pose a problem here.