In earlier updates, all three components were buyable, but now only chassis and gearbox can be bought. And if you take over another company, all their engines are set as 'discontinued', even when the vehicles that use these engines are still in production...
Welcome to the forums.
I've noticed this issue as well.
I did some tweaking to the AI in 1.09 which I have retweaked since. The AI will not prematurely kill off engines now.
However I have ran into another issue with the AI engine system. I'm currently looking into how to fix this.
As such I have marked this thread as Fixed. Thanks again.