Do you have a volt / ohm meter to check circuit paths?
I take you are jumping the large battery + solenoid post to the other large post that goes to the starter. Have you jumped the large battery + post to the small (third) solenoid post (might be a orange wire - not sure)? If it cranks that will tell us that there is an interruption in the key start circuit, possibly including a safety switch. The engine may not start unless the key is in the run position.Please be careful overriding safety switches and starting. That may result in an unexpected movement and injury
post make model and serial of unit and I will try to find a schematic.