Don't panic! Seriously, one of the most "unlucky" pilots in my club can get the Rascal to fly on the stock motor system. It's the biggest patched-up, bashed-up plane you ever want to see, but when he gets the hand launch right and doesn't overcontrol on the elevator, the plane is a nice little flier. He gets up around 400' and floats around for 10 minutes or more on NiCds.
This rumor that it doesn't fly on the stock power system is pure BS. Of course it won't hang on the prop or climb straight up, but this is a RASCAL fer cryin' out loud! What do you want?!?!
Definitely run 8 cells on the battery, and use light radio components. If it's the kit, build light. Don't gob on the glue, don't use regular Monokote/Ultracote. Sand sand sand sand and sand some more. Get the balance right, and keep the elevator throw down. Set the gear lash properly by sandwiching a strip of printer paper in between the gears, pushing them together snugly, tightening the motor down, then removing the paper. You can waste a lot of power in that gearbox if you don't set it up properly.