Reddit reviews Uni Pod Filter 28mm I.D. x 76mm Length UP-4112

Man, a 125 down on power? That's a hard life!

I would ditch the box and get this OR this

I can't find if your carb is Constant Velocity type though or not. If it is, a pod filter WITHOUT rejetting is a bad idea, and CV carbs NEED metered air to go into them, and the box meters the air. But pod + rejet = more power sooooo take it for what it's worth

Should be a 28mm intake, but you might want to measure just to confirm.

Cleaning the fuel tank = draining out all the gas. it hasn't sat for SUUUUPER long so just getting rid of the old gas should be enough. Once drained, pour in some new gas, slosh it around, and drain that out too.