Walk behind Honda Master Series HR 215 SXA Transmission Issue

I am the original owner of a Honda Master Series HR 215 SXA. I believe frame serial number is 6211077 (on aluminum tag behind engine on deck). Over the past 3 years and continuing to get worse regardless of what gear I am in (1st, 2nd, or 3rd) especially when engaging the self propel or going up an incline the transmission skips/ratchets trying to engage the rear wheels.
Once the wheels engage it works fine. The problem is not the ratchets on the ends of the driveshaft connecting the wheels. I have removed the tranny and separated the case. Now I am needing help on what to look for to figure out which gear is most likely worn out/skipping. Also what gear oil do you guys put back in after fixing them. Where do you find replacement parts. Thanks for any help.

Newt
Omaha NE

http://www.hondapartsnation.com/oemparts/a/hpe/505ce407f870022d24bdbcc2/transmission

I believe these folks have the best prices for HONDA parts & ship fast to your door. My oldest son has HR215SXA that i purchased new in 1990, uses it weekly. I have replaced the transmission twice over the past twenty seven years, price of transmission has doubled since last purchase. It's still less expensive than a new Honda mower. You may want to carefully inspect transmission parts for wear & just order worn out parts. Specified oil: SAE 90 hypoid gear oil, oil capacity 0.21 U.S. qt.

Model numbers do not translate directly between USA & Aus but if that is the 3 speed look at the pin inside the box that changes the gears.
It is always spinning and the lever does not so it wears and a little wear will prevent it from changing fully.
The other problem is the clutch dog but I do 1 of there for every 20 pins