That's a very nice looking bike.
BUT..... (you know what's coming next
Unless you are ABSOLUTELY sure that its been done . . . Strip the motor down and clean out the crankshaft sludge traps. I used to strip my motor annually for this purpose - and I always changed the oil at 1,000 mile intervals.
There are lots of threads (and horror stories) on this forum around this issue so it isn't a myth.
Better safe than sorry
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH