The 900T Amigo in the Alps. ???
I have a completely overhauled GBS engine (from a 850 sport/spider - 54 hp vs 37 hp original), my experiences so far (our Amigo is fully loaded with 2 adults and 3 children. Also a 250 kg luggage carrier):
- yes you must install/leave the original low gearing diff. It's attractive to use a long diff from the sport/spider, but in the mountains you mostly drive in 2nd or 3th... on the highway the Amigo is happily revving and cruises 95 km/h easily.
- no serious heat problems occured, but you must have a good water pump, original fan, new radiator and all(!) the baffles installed. Also pay attention to ignition timing and don't set the carb to lean. I installed a water temp sensor with gauge from a 900E. Nevertheless the watertemp almost reaches the red zone on a steep / long way up. Today it was about 33 degrees outside.
- my engine has a compression ratio of about 10.5 to 1. Uphill when going strong sometimes the engine slightly knocks. It was all over with BP 102 Ultimate. No normal fuel in the mountains.

Interesting comments, I've obtained a few 'performance' bits as part of my restoration that should (in theory) boost the BHP on Connie from 35 to somewhere in the mid 40's.

One of the other things that I'm considering is to add an in-line octane booster to help compensate for the lower octane in unleaded fuel. There are loads of claims on how good these are but has any one in the club used one and if so what do you think?