@LobusLives - There are few modern synths which use Chowning FM Synthesis, probably for good reason. When the DX-7 came out, it took over the industry. However, within five years or so, FM synthesis became not much more than a memory. This is because FM programming is difficult to master, and so different than analog. Also, there was no way to get any good training on FM in the day. So, FM was pretty much abandoned. Even today, there is very little on FM programming. Chowning's and Massey's books came out around the the close of the FM digital era. Most had moved on to wavetable/sampling synthesis by then. It is a shame because FM can do so much amazing sound.
The only synth I know which is doing serious FM is the Montage. I understand that the P-12 has upgraded its firmware to allow linear FM - which I do not know whether it is equivalent to Chowning's FM or not. However, I do not have any experience I related to the P-12. I did buy the P-12 module on Black Friday. So, I can probably update you by Christmas. The problem with the P-12 though is it only has 4 oscillators albeit them allowing more complex waveforms. If you approach FM with sine waves like Chowning did, you will be down two oscillators from the DX-7. So, I doubt the P-12 will be a true replacement for the DX.
Since you are going through the Chowning book, I would suggest that you get a first series DX-7 so you can do the exercises - even if you decide not to use it as you primary FM synth. (MKII and Grey Matter not needed). Those exercises are really important to understanding and ultimately mastering FM. Massey's book also uses the DX for the exercises. AFAIK, those are the only two books out that are practical training manuals for FM. Once you get the basics of FM under your belt, you will be more equipped to decide what synth will fit your requirements.
Personally, I still like my old DX. It is capable of some really amazing sounds. For FM, my upgrade will be to the Montage. However, given the cost, it will probably be a little while before that happens. This upgrade will have the added benefit of upgrading my Yamaha S90. so, I will be looking at the Montage for more than FM.