Okay, this may be a crazy idea but I have always wanted a 61 key hammer action keyboard but they are scarce. I was thinking of buying a motif ES8 keybed and using 61 keys to replace the keybed on the motif ES6. Is this likely?

It’s not a crazy idea, it’s just very impractical.  ( I never say “impossible”, because somebody would do it just to prove me wrong.  :-) )

To get an idea what the differences are between the ES6 and ES8…
https://elektrotanya.com/yamaha_motif_es6_es7_es8.pdf/download.html

Perhaps you can find a used 88-key hammer action controller, or even a damaged 88-key synth where the keybed, PB, MW, and 5-pin MIDI-Out still function. It shouldn’t matter what other functions might not work (including the tone generator), and the brand should be unimportant as well. The controlling device and your ES6 could then be connected via MIDI, resulting in 88 keys of control and an ES6 that was still intact and functional.

The problem is the keybed height.  The mechanism on the Motif classic/es/xs/xf 8’s are different heights from bottom to top vs the 6/7. 

To see what I mean, take a level-on front look of a Motif of any generation 6,7 and 8.  You will see the 6 and 7 are noticeably thinner, but the 8 is much taller to accommodate the weighted keys.