It was made in 1900 and the sn has no significance. Proofhouse.com has Colt sn data.
The engines are a totally different design but the crankshaft design of both can be traced back to 1958 at least. The 300tdi and the 200tdi will fit your gearbox except one bolt which you leave out, you will need to move one engine mount and make up the exhaust and wiring loom to suit. The 300tdi will not fit behind the grille of a 2A without a lot of custom plumbing for the radiator. The 300tdi block is longer than the 2.25l or 200tdi motors. Ideally, the 200tdi is the correct retrofit for a 2A.
It is (1500 + 2000)/2 = 1750