Well… The x90 Mk1 radios have a carrier that holds 8x C cells. This can be removed to give a fair amount of space. It would be easy to make up a 4S LiFePo pack to fit inside with plenty of space, just simple bit of 3D printing. What you need to watch is to make sure you disable the NiCd charging circuits built into the set and probably have a series Schottky diode in the new battery +ve to stop any back-charging from an external supply. The x90 MkII sets have the removable clip on battery pack which has slightly less volume and a different internal shape than on the MkI so the 4S LiFePo pack would be different.
The question is why? They’re 40-50 year old designs of average performance. Sure it’s nice to be able to keep something out of the landfill and as I said, you do not get their QRP output to anywhere near their full power unless the voltage is 13.8 and not 13.2 or 12.6 or 11.1 volts. If you want to make an 18650 or 26650 4S LiFePo pack, do so and use it with a flying lead and keep it external so you can use it with any radio. (Remembering the x90 has a backwards wired barrel connector). You could also do like Rick M5RJC did and use up scavenged vape LiPo cells as he did here: 817/8 modern battery solutions - #3 by M5RJC
There’s a case for using them to drive uWave transverters where their deficiencies are less of an issue. Such as G1DFL’s 76GHz system here:
(c) G1DFL
I wouldn’t be looking at using them for 2m/70 work when a 705 with its filters and DSP is much better. And a lot more expensive but you get a fairly tasty HF radio too! The only advantage is their low current draw on RX. Even then on my recent 2/70/13 activation in the recent extensive tropo conditions, I only used around 50% of a 4Ah LiFePo with a 705 running mainly FM flat out for 2.5hr.
So yes you can but that doesn’t mean you should. Looks about shack and wonders why there’s a TR2300, FT290 MkI, FT290 MkII, FT790 MkI and an IC202 looking for a use 