I did a ton of research on coolant that would be compatible with all the materials in our scooters. I read every coolant bottle label I could find, figured out each ingredient and the effects those ingredients have on the materials in our scooters, and went pretty deep down the rabbit hole of coolant chemistry.
Come to find out, Valvoline Zerex Asian Vehicle anti-freeze is the best thing out there for our scooters.
http://www.kymcoforum.com/index.php?topic=3649.0Also, as regards filling the radiator... you can fill it, leave the cap off, start the engine and fill as necessary to bring the level up. Snap the throttle open a few times to flush out the air bubbles. Don't worry, you won't cause any coolant to burp out. I've done it.
What I did was drain out the old coolant, fill with tap water, idle to mix it around, drain it, fill with tap water, idle to mix it around, drain it, fill with distilled water, idle to mix it around, drain it, fill with new coolant, idle to mix it around, drain it, then fill with new coolant again. I ran about a gallon of liquid through in doing so, but I guaranty there's no old coolant in there.