Not sure on the sprocket, but I suspect it is stock. It was rebuilt after the previous owner ran it using straight gas. It may not be as fast as a new pro saw but it seems to cut anything I’ve ever attempted to cut. I may be wrong but I thought I read somewhere that it originally came with a 20 inch bar but could run a 24 inch bar.
You got me curious, so I did a little digging. My 036 PRO is 63cc, and typically was most often purchased with a 20" bar. But honestly, bury that bar nose-deep in red oak, and it will slow a bit.
I found I could make it work much nicer by putting big felling dogs on it, truly larger than anything such a small saw deserves, but it did the job of reducing the exposed bar length to 19". Now it doesn't bog when I bury it in a 36" diameter oak log.
The maximum bar length spec for a saw is usually based on the capacity of its automatic oiler, not the horsepower of the saw, as variation in wood species and chain type and size will dictate different power requirements the same bar length. Also, the fraction of RPM drop acceptable to one user, may really irritate another, expectations vary. Point in case, the aforementioned 036 PRO is rated to swing a 24" bar, but I'd never do that in hardwood.
Dropping the chain speed by running a smaller sprocket can make a big difference, since it takes less horsepower and torque to spin the chain slower thru a log. This is why your father got away with running 20" and 24" bars on some very wimpy saws 40 years ago, they had low-end grunt, but not much speed by today's standards.
Doing the math, the 63cc 036 PRO runs 13,500 RPM, and the 65cc Jonsered 2165 runs around 12,500. So, if I find 19" is the upper end of the 036 in oak, I'd guess the 2165 would be pretty comfortable up to 21 inches, in the same wood. If you only cut softwoods, or run a narrow chain or skip chain, then likely 24" is totally acceptable.