Reply To: 125 vs 250 VTwin engines
No need to upgrade the oil pump, changing to 25o cooler is the #1 engine mod for the 125cc , your rear cylinder will thank you!
Front cylinder not a problem as it gets a lot of air anyway.
Sprockets, try a 520 chain conversion kit (52T on a 428 chain rear wheel is too big for long distance riding) , 520 sprockets have 46T rear or 48T (take it from GD250R sprocket will fit)
If you keep 428 chains, try 15T front and 52T rear (Can’t change size on rear on 428 chain sadly) but if you don’t care about acceleration, 15T may help reduce the revs further for more speed
Don’t forget new clutch , new springs , and valve clearances go a long way , use the service manual (GV125 and GV250 are the same clearances as both engine use shims)