Step 1) take care of air flow:
hack of as much of the top of the airbox as structurally possible, next take out
the baffle on the pipe. (I'd get an aftermarket pipe for sure, but nothing fancy)
Step 2) re-jet:
Step one will make you bike leaner so unless it's already too rich it will need to be re-jetted
if you've ever done it before its not hard but you need to know how a bike is suppose to behave
and you should have some basic skill. You should find someone you know that has said traits.
Jet kits are cheap under $50 in most cases.
Step 3) Gearing:
making the sprocket bigger will make it wheelie better but you will loose top end speed.
A "race" chain is a chain without O-rings it wears faster and requires to be lubed more but is
lighter which will reduce rotating mass and get you more HP to the ground
Step 4) Maintenance:
keep your tires fresh with nice sharp corners on the knobbies, clean air filters
(once you complete step 1 your filter takes in more air faster dirtying
it more and faster) keep you
chain tight to avoid sprocket and chain
Imo "rev boxes" are a waste on such models ass the TTR they dont change any ignition timing all they to is give you a higher rpm allowance, which is usless on fourstrokes becuase you reach peak HP way below peak RPM. If yous has electric start you can always remove the battery and any other electric start componet possible to loose weight. (as long as it has a kick starter too)
As far as motor work goes anything other than a simple rebuilt is a waste of money, bore kits and such are a waste and will lower the likley hood of reseling the bike. (dads usually by these bikes for sons and if its a frankenstein they will assume it has been beaten)
I used to have this model of pipe on my old crf 150 its a great model really loud thought that was my only complaint, http://cgi.ebay.com/ebaymotors/New-FMF-Powercore-4-Exhaust-00-07-Yamaha-TTR-125-L_W0QQcmdZViewItemQQ_trkparmsZ65Q3a1Q7c66Q3a2Q7c39Q3a1Q7c293Q3a1Q7c294Q3a50QQ_trksidZp3286Q2ec0Q2em14QQhashZitem3ef60e25a0QQitemZ270416094624QQptZMotorcyclesQ5fPartsQ5fAccessories
Ketchup not just a condiment BUT A LIFESTYLE.-steezeeepoliz...
they face........it shall be hurt.-beezey
reppin the 5ive 1ne 8ight