slipping avante 2001 centre diff

[mattf77]

Hi just cleaned up and rebuilt my avante 2001 and the centre diff keeps slipping, with a 25 year old brushed motor. I've tried tightening it up and rebuilding but still slipping.
I was hoping to upgrade to brushless but it cannot even handle the old motor and a 25 yr old nicd.
any advice on replacement parts/ alternative gearing to be able to put in a brushless set up, or even just get it going as it is?
cheers
matt