Chainsaw Repair

Would the Koyo BH-108 needle roller bearing full compliment drawn cup with a 6200 maximum rotational speed work?

Koyo makes very good bearings. They would be fine.

Torrington was sold to Timken in 2003 and sold to JTEKT Corporation in 2009
The Torringtons were only rated for 4200 rpm.
They have held up for a long time in those saws. I doubt you could find an occurrence of a failure

If you were to research the info, lots of the seals and bearings used in saws are run beyond their rated maximums with no issues.

I suspect the low loads put on them are the reason they are quite capable of standing up for many years.