Howdy
Thanks
Well, wait to see this week how will be with the snow, because for two days will be snow at 3000 feet (maybe just Mt Hood) and in the end the week will be another round of two or three days of snow and after that you can decide if you can leave. I believe it's not to late to do the trip. Only money is stop me to do the route 5 this year.
Usually in the end to the October can be expected snow on the trails around to Sisters. Plus, most of the trail will be up on 4000 feet altitude.
When I was camping near with Sisters, the temperature over night was in 20's and in the morning the ground was frosted, but was easy to drive over ice roads. Definitely your cruiser need to be prepare for cold weather and care a tarp with you to cover the hood over night. Will help the engine to stay warmer over night. I did like that and I felt the difference between the night when was in 20's and I didn't covered the hood and when I covered the hood on next nights.
PM if you need more info about rd condition, detours, where you can get gasoline and other info.
good luck