Bit difficult to be positive without the engine numbers. Things like electronic ignition and the later type BVF carb you mention could just be additions to the original engine. However, my guess is that you have a later engine than 1987 as electronic ignition was not common until post 1990-91 and even then came in three flavours. from memory the spring type carb was also something introduced later than 1987 but not sure exactly when.
if the head/barrel and crankcases are black rather than raw alloy, this would also suggest a later engine. However, there is no difference in the basic mechanics nor in the lubrication system and in my view the carb you have fitted is the best of the BVF options. The final fling was to use a Bing carb but you also need a different manifold forthat option.
Hope that helps. I don’t often follow threads on the website so if you want more info email me direct.