“they’ll have to dig out his remains…” Google translate is giving me “Yo pral oblije fouye soti rete l” I’m thinking of using the creole word “ko” for remains: “Yo pral oblije fouye soti ko li yo”. Is this correct?

Here, to dig out will translate as "detere" (to unearth)
If Google Translate is not giving you the right translation, try using other words, in this case, "cadaver" or something else.
Here, remains will translate as kadav or zosman

They'll have to dig out his remains. - Yo pral detere kadav li. or Yo pral oblije detere zosman li.(if the body's been buried for a very long time)
and...
No need to use plural "yo" unless you're talking about bones.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words