Thanks for any and all advice. I have been searching online for leads but have come up empty.

2009 Big Red with Camoplast Tracks
Have been using to plow snow but power is way down from year's past. Still have plenty of torque to push snow but for whatever reason the machine will not shift into second or higher. Also very slow going up the driveway.
I have changed the plug, and the oil and filter, and the air filter. There is no check engine light or codes.
Machine sounds great and there is no smoke.
There has always been some missing/misfiring at high RPM in reverse, but now I noticed some at high RPM in first.

Things I still need to do - adjust valves , change diff fluid

Any other tests I can look into?