oil cooler install?
 

how would this oil cooler be installed on a QMB139 50 cc?

http://www.monsterscooterparts.com/oil-cooler.html

you would have to drill and tap the high pressure side of your case, unless you have the rare cylinder thats tapped for it already. theres kits that plug right into your oil drain bolt bu they are around $200

no reason to install an oil cooler... rather buy a more expensive and efficient oil... like wnb oil for racing

REPSOL racing 10w40 is the nearest oil to moto racing oils

Having a cooler means your bike holds more oil. Plus, the pipes to and from.
More oil means, not only with the oil stay cooler, so will the bike. And it wont wear out as fast, so you can wait an extra 1,000 miles or so between changes.