Which engine is this?

Recently came in to a late series 2a but I'm pretty sure it doesn't have the original engine.
I think I have found the engine number towards the back of the engine (closest to windscreen) on the left hand side, and it begins with 12J ..... which would make it a 90/110 2495cc engine (using https://www.glencoyne.co.uk/enghist.htm ).
Any advice, tips, things to look out for on this block?




I have attached a couple of photos

Thanks in advance!

That green colour on the block might imply its ex military.