Danman, not to dispute Psycho's advice, but getting rid of the airbox is not that great an idea. For one thing, you lose any protection from water and mud. I really doubt that a 175cc motor in playbike tune will benefit that much from a foam sock aircleaner anyhow. My advice is not to butcher up an old bike trying to make it something it never will be and never was. You've got a small play bike, so play with it. If you want to race, get a race bike. You'll be money ahead and more satisfied with the results. That's my 2 pesos' anyhow.
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'81 490 Maico (King Kong!), 2000 CR250, 74-1/2 GP400 Maico, buncha street stuff