I have already trodden this path with a Tri-ang EM2. My answer to a rebuild was as follows.
The body was been detailed with seperate handrails, lamp brackets, etc and the moulded lining carefully removed. The chassis is a heavily modified Bachmann class 37, with new bogie sideframes constructed from scratch using plasticard and other bits from the junkbox, as the Tri-ang models has one cast, and one plastic bogie of only moderate detail. However by using the class 37 as a base, the new bogie wheelbase remains, like the Tri-ang model, a little too short, but with careful arrangement of the bogie centres this is not too noticable. No RTR chassis seem to possess the correct wheelbase measurements. The Pantographs are from Judith Edge, and though fiddly to build, they give a more realistic appearance than is given by using other ready assembled H0 pantographs.
I enclose a pictue of my conversion, and I have placed further views in my Gallery 'North Eastern Electrics'