How would I find a serial number for my 4bt

I may be able to help you some. CPL 0858 was a very common one used in road vehicles. There were different specs on certain parts depending on application. One version came on the Ford E350 vans. Generally, you will see it with a top mounted turbo and a belt driven vacuum pump. It would have a transmission adapter with a small block Ford bolt pattern and a flywheel for a manual transmission. The GM units came in 2 designed. Both those version will usually have the turbo mounted in a low rear position. That turbo tend to have a different sized turbine housing. Normally, no vacuum pump on those. The transmission adapter is a standard GM on both versions. The difference is one come with parts for the TH475 that was found in GM P30 van. Maybe 10% of the vans had a manual transmission and those have a GM flywheel. Have you got a photo of your engine? If you can tell me which engine style you have I have ESN's for them. If the data plate is missing from the front gear housing, the engine serial number (ESN) is sometimes stamped on the rear of the block just below the head. This happens only when Cummins has rebuilt the engine. You mention the CPL and the injection pump can give a clue to that number. CPL 0858 changed injection pumps a lot over the years when it was produced. There were about 8 or 10 different numbers for that CPL.

That number on the engine won't help. Which injection pump do you have? From that there should be an ID plate on the pump that has a series of numbers. From that we can probably narrow down the CPL of the engine and find you a serial number you could use to buy parts. If the engine has been rebuilt you would need to measure rod and crank journals to be sure what size bearings are needed.
Here's a photo of a VE pump where you can see the ID plate.

Here is one type of ID plate.

You see the number there 394 6371. That is a Cummins part number 3946371. Gives you something to look for.