Mike:
I had similar question about the end of our Village Italy tour and asked the RS Tour Department for some help. According to the response I received from Heidi of the Tour Department:
1.) You can take a train from Orta to Stresa (involves a change)or take a taxi. She didn't mention the cost of the taxi.
2.) There is a bus from Stresa to MXP (Alibus). There is also a way to get there by train which also involves a change.
Heidi recommended that we visit Stresa for the day since there is more to do there.
Not sure of the date of your tour but it looks to me that the bus only runs to the end of September and our tour ends in Orta San Giulio on October 4th. Heidi thought the bus continued beyond that point albeit on a reduced schedule. That's when we starting looking at the train alternatives. If you hit the connections right it looks to be about a 2 1/2 journey to MXP. It looks like there are no easy connections to MXP from either Orta San Giulio or Stresa.
At this point we are unsure whether to stay in Orta San Giulio for an extra day (our flight leaves mid-afternoon the next day), travel to Stresa for the day or head to Milan.