In my trialling days in a Suzuki SJ, small and light but famous for falling over, I found a 40* side slope was no problem at all if I was sitting on the uphill side. The other way round it could only cope with 30* before I would end up lying down in the muck. Haven't tried it in my 70 cos there's no roll cage in that and I couldn't bear the cost of mending it after I found out the limit.
London Transport used to test out their buses on a sideways tilting platform. Amazing how far they can go before falling. I wonder if their old machine could be hired out for your 90?