David,
I searched the web with almost no success
I couldn't figure out what engine Hanno used on MY 90
even I think he never used a 90 2s because of the competition rules of those days ( if i'm wrong please correct me)
In my search results I only gained the same info that every body knows
It seems that MY 90 refers to 90 4s that could be replaced with a 60 2s engine and maybe it's because 4s engines were getting more popular those days.

Regards,
Ehsan