Hi Jimbo,
I most probably built mine about the same time as your father bought yours.
Unfortunately I did a very regrettable thing, a few years later, with empy pockets, I had to sell it, and regretted it from that day to this. To me it was one of the most realistic sailing models I have ever built. I did buy another kit when one became available, the old issue, not the new. Unfortunately it is sitting in my loft untouched, as I will never be able to build it.
But to get back to your problem.
When I made mine, I thought the C of G would cause a few problems because of the fairly high superstructure and a lot of brass wire, coupled to a fairly shallow draught. So because of this, I set the battery (6 volt gel cell), on its side, on the bottom of the hull. For power I used 2 x 380's on the outboard tubes and a 540 on the centre. I fitted it out so that it only used the 380's normally, but at the flick of a switch the 540 was brought into play to get me out of trouble. The boat does look unstable and heels a lot when turning, but this is normal and is why I liked the boat from the very beginning, because it simulated the full size just right. There is only one major drawback with the hull, and that is when going in reverse at speed, if you haven't got a good deck seal you become the owner of a very fast submarine.
If I was to build it now, I would most probably go for 3 x 12volt 385's, all working in unison. There are a lot more modern motors available now, plus I have learned a lot since I built mine.
I have included a few German sites for you to have a look at. Don't know if they all still work correctly.
I hope that this info has given you what you need, if not ask away and I will try to get my brain cells working again to remember exactly what was done.
http://209.85.135.104/translate_c?hl=en&sl=de&u=http://www.ig-schiffsmodellbau.de/wiesel.htm&prev=/search%3Fq%3D%2522schnellboot%2Bwiesel%2522%26hl%3Den%26lr%3D%26sa%3DG
Translated version of http://www.marinemodell-fotoarchiv.de/fotoarchiv/flottenparade/flottenparade2002/Deutsch/wiesel.html
Translated version of http://www.fmg-bln.de/wiesel.htm
Translated version of http://www.u-boot-modell.de/modellbau/wiesel.htm
John