Hi,
Check the parts manual again, but I'm sure that ALL oil injection BS90s and BS100s use the same cover. The premix bikes used smaller covers (the 90 Sport had a dealer cover to allow for the bigger carb, and the 90 Deluxe, Trail and Mountain used the 50/60 Sport cover.
The correct oil injection 90 cover is# 2176-5020 and is quite tall compared to the premix cover - best thing if you're not sure is to compare the shape of the cover with the shape of the carb/oil pump chamber on your engine.
Hope that helps,
Graham