well again your tire pic, that says KT 128,
this is the model of the tire, NOT the size!
the size again will consist of a set of numbers like this
an example again LOL
if it said this,
25x8x12 on front, and (then normally slightly wider on rear, 25x10x12)
the 25 is how tall tire is,25 inches
the 8 means how wide it is,8 inches(or other example would be 10 inches )
and the 12 means the wheel size the tire goes on!, a 12 inch wheel/rim
as for the things your stating about is basically runs OK
well that is good
but if it is having some issues idling or running, , normally these issue's are fuel related
fuel in a tank can go bad fast, or in these older atv's, the tank over time can get dirt and what not's in it
I get your a kid/younger person, and good for you for wanting to learn
IF you wish to learn more
what I would suggest is getting yourself a service manual on this atv, there are free downloads available
and used hard copies can be found ion places like E bay for fair prices!
it will be something you can use to learn a TON about this atv and how to make it run its best!
NOW< if you wish to try and make it run better
you can do some basis things
one drain and fill tank
make sure peacock is working and free flowing in run/reserve
next you can replace fuel line from tank to carb
and last, you can remove carb and clean
doing this will most times cure more funny idling running issue's
there are TONS of Youtube videos out there to watch on how to do all this as well, which might make life easier on you to learn and do so
I also suggest taking pictures as you go, or video or both, place any parts as you take off in plastic baggies and mark /label them when ever doing repairs, will again, make your life easier!
as for a gear reduction, I will gather this is PAST your skill set and ability, as it requires larger tear down to remove it