Cycled it about 3 years ago now. It's do-able and one of the best climbs I've done in Europe. Recommend you get a good tail light and a reasonable headlight.... there are a few tunnels that are 1 lane, dark and wet. you'll need the lights to be seen by the cars/motorcycles more than to see yourself.

Bring warm weather gear. the weather is dramatically different from wherever you start (I started in Bormio 18C and it was snowing 6C at the top). There is a good restaurant to warm up in at the top.
 
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