I wouldn't think a 90 Etec would fit a tight budget.....
I think the 75 had the same basic parts as the 85/90 could be a carb change a quick skim of the head will get you in the ball park.
If not if you post a wanted at
www.lobmbc.com in the for sale section attention Pete Mantripp he should be able to source you a decent Yamaha 90 for resonable money.