I installed this 17T front, seems to work well....
http://www.ebay.com/itm/151719928903...%3AMEBIDX%3AIT
Also purchased this 45T rear, not sure I need to install???
http://www.ebay.com/itm/161739496711...%3AMEBIDX%3AIT
The stock carb seems to be working well for me, FWIW....
The front sprocket change is easier and cheaper than the rear with no chain cutting needed, the 17T front for $9 is an easy job, and may be all that is required. At this point I'm uncertain if changing the rear from 50T to 45T is warranted and will be a benefit. I hate to go through the effort to change the rear, shorten the chain just to have to "undue" it...
The oil "filter" is just a mesh screen cup that is reusable, it is behind the large nut / cap on the low side of the engine.
Hawk