91 will still work well. I doubt you get the high summer temps up there. I've actually ran 89 on my evo build with no problems at all with the Wiseco pistons10:1 and still got great fuel mileage...

 

It gets plenty hot here in the summer. 90s are common, near 100 or slightly over a few days each year.
I can run 89 in my TC95 with 10-1 with no problem.
Bob's evo is a rattle can with anything less than 94.
It runs out great and is very good on fuel, not so good on oil.

 

Wow.... Wouldn't think it gets that got up there. The only time i would have it bad was when i would go into Manhattan....Learn something new every day... Let us know how you make out.....

 

The heads are out at a shop getting checked over.
KB920 pistons in +.040 are on the way.
I rebuilt his front forks and replaced the steering stem bearings.
A new Russell front brake rotor is also on the way.
He had way too much timing in the motor, the Crane hi-4 was set for advance curve #6, explains why it needed 94 fuel.
The clutch was badly in need of adjustment, the pull feels like a new bike now.
It's a dirty filthy neglected bike, covered in engine oil mist and fork oil.
When I get it back together, i'll probably spend a day or two cleaning it for him- Just 'cause I have way too much time on my hands.