information wanted

I have recently purchased a 1981 French registered lhd VW Golf Cabriolet which I think is a Mk I but has very little documentation.  The French registration document gives the VIN (WVWZZZ15ZBKO 18486) but no other technical information except for the nominal HP (7) which I think indicates an engine size between 1500 and 1750cc. I have noted the engine number (0251033730) and want to find out if this is the original engine or a replacement. Also if I have the engine size I can find from the Haynes manual the engine code (2 letters) which I can't find stamped on the engine so that I can check if I have the right carburettor and spark plugs etc.  (It smokes heavily for the first 10 km but then appears OK). There are two other codes stamped or plated under the bonnet, one of which might be a chassis no and one is a type no 15 5J B1. It is also fitted with what appear to be special wheels and a tyre size (185/60 R 14-H radial/rotation) not given in the Haynes manual so I don't know the correct tyre pressures. (I've chosen 2.0 bars front and rear). It seems to run OK and I like it very much but selecting gears is still a bit of a mystery, with excessive vertical movement on the gear stick and occasionally an annoying whine on 5th gear.
Volkswagen UK and the VW owners club haven't been very helpful. I don't speak German so can't easily use the VW Euro site.
Other than learning German, how can I find the information I need?

The engine number you have quoted looks to have too many digits. It should be 2 letters then 6 numbers from memory. On big block engines (over 1.4 litres), the engine number is stamped on a flat section of the engine block, front right hand side above the distributer. It will probably be difficult to read by now! Otherwise the engine type (2 letters) is given in the service book and on a sticker inside the boot.

Failing that if you post some pics we can tell you approximately what the current engine is.