You can crank it all day with a drill and it wont start. The engine is fighting the drill, it wont kick off.
engine requires :
air , fuel , spark ,
reasonable compression (atleast to start)
good compression for performance.
look at the simple things first ;
air- is filter restricting flow
fuel- fuel filter pluged/ dirty ,
fuel pump working,
flow from tank to pump to carburetor,
carburetor dirty/restrictions internally .
spark - spark plug ( fouled , wet/ oily)
compression - vavle/s maybe open-
[set lash] or damaged, timing is off,
bad head gasket, broken piston rings,
hole in piston, scored cylinder bore .