Weight in the tender isn't the issue. There's plenty in it and I have a drive that I can fit on the centre axle but it would also require an enlarged slot in the base cover to accommodate it. So the motor issue remains relatively easy. The problem is the tender and its associated tender drive because a new axle base would be needed and , here's the punch line, the streamline tenders on the those models will not fit a standard base. They are too long for the standard Stanier tender. Nor will I require much weight in the tender, it'll need to be in the loco to get the adhesion right. But If I manage to get another one [loco that is] I might have a play around. They're not worth that much and unboxed. , As it is I have the nearest to a prototypically correct Hornby product so I don't fancy the butchery on these.Tricky one as you'll need to dismantle the axle to get a worm gear on. Probably easier to replace the tender wheels with some non-traction tyred ones and add some weight to the tender.
I use lead flashing left over from an old roofing job for my models.
As it is with the 2P and the 3F that I have which are body drive, yes they slip, I like that. Since I'm not on DCC or all that I can have fun double heading over the viaduct.