Any of the newer Yamie synths have a lot of the DX7 sounds incorporated in their XG extensions.

I would suggest the Korg N1R. It has the
Yamaha XG Sounds
Roland GS Sounds
Korg Trinity Sounds
Korg XGProX Sounds
Korg 01/D Sounds
Korg 05 Sounds

all in a single rack space.

This unit is under $400 now.

You could also try to find the Yamaha FB01. Itis half rack andhas the same types of FM sounds. These sell for under $100.