[Ggmac]

Could be stuck rings / valves . I'd stick new oil in it , add water and fire it up . Run it for 1-2 minutes, shut it down , check oil water let cool and repeat . Do it a few times , see if the compression comes up . Heat and movement should give you the answer you need .
If you hear any thunks shut it down .
If you don't want to do that. At least remove the intake and heads , examine and check condition. If it all looks good reinstall and go back to first suggestion of running for a minute or two .