We've actually hit 811 distinct aircraft models/engines in our database. Everyone thank Dex.
I've actually taken the quick time it takes to go into excel and make a lovely chart of how many aircraft are available in any given year in-game. Distinct Engine Variants is the total number of options available (all types, all engine variants), Distinct type names is what it sounds like, it rules out any same-name engine variants.

Our peak year at present is 1976, where we have 76 manufacturers producting 308 different engine/aircraft variants, or 283 different distinct models of planes (if we rule out the direct name engine variants).
We have the most manufacturers available in game in 1968 with 78 manufacturers online in that year.
It's also interesting to note there are now a total of 124 manufacturers in game.