When I was there two years ago I was there for a full week and stayed at Alla Torre Del Barbarossa (B&B), I would say if you have the chance, make a reservation with them to get diner there since the food is absolutely amazing.
http://www.allatorredelbarbarossa.it/home-it/ You cant just walk in for food since its not that big and it is always full. THe owner (and cook) told me some stories about him riding on his V50 to Portugal and such and according to some people I know that speak also speak italian thats also a place where some old guzzi engineers occasionally go.
If you just want some simple pasta made delicous go to: Il Canton Della Pasta @
https://goo.gl/maps/Er2N7, that guys has some awesome bikes as well but not at his shop usually.
Other than that, don't forget to talk to the elderly over there, talked with an elderly lady when I was there who worked for guzzi during the second world war. Aparantly it was supervised by engineers from Zundap (if I remember correctly) and they were also making parts for panzer tanks back then. Other than that everybody is very warm and friendly in Mandello. I found the people that were more in the touristy places were way more closed off like at Bellagio or Lecco.
*edit*
don't forget: Motofornitura Valassi Di Conca Silvana & C. (S.N.C.) @
https://goo.gl/maps/pRWzH
