The track assembly is from an OC-3 Oliver and I think the rearend is too by the looks of the steering levers. If the original engine is gone so is the serial number because most likely the flywheel housing has been replaced or altered as well.  The serial number plate should be on the right hand side of the flywheel housing and in plain view if the original housing was used somehow.
The OC-3 was built until 1957 and parts for the Oliver parts are available from Zimmerman Oliver-Cletrac 717-738-2573, but I doubt they will have any engine parts.
I would be interested to know more about this crawler and what make the engine is. I think a Kubota would make a very nice retrofit in an OC-3 and this looks like it wasn't cobbled together.