Just done this conversion myself. Chop out as much of the original ballast tank as possible ( just leaving the 'sides ' of it ) and plug the ballast holes with styrene strip / glue.
The battery ( 6v 4.5ah ) will sit perfectly on it's side in the now defunct ballast tank. The battery is now your ballast. ANY screws you remove from the boat put back exactly, they are all slightly different sizes. I ditched the water pump and fitted a Graupner one. I run using a Zebra 3 channel radio, the 3rd channel operates a servo to microswitch to pump motor. If you remove the rudder screw from the black disc moulding to release the Kort nozel DO NOT over tighten it later as it will snap the fitting ( you are only screwing into plastic ) My original motor burnt out because i put too big a prop on it, i now run with a MFA 385 type motor, 30mm 3 blade brass prop ( changed the prop shaft but you don't have to ) and mine now runs at a scale speed, the motor is barely warm and me and my son's get bored with over an hours sailing and that with the pump working for a lot of the time so in effect you are actually running two motors! Any other help you need? No problem. Just remember to not try and fit the largest prop you can find, it does'nt work that way. The original motor is fine, why nor just paint the prop with some brass / bronze coloured paint. Regards, Dave.