I have not used a "Smoky Joe", but have used a Hornby "Ben" as the basis for a model based on Heywood's "Katie".
For "Katie" the wheelbase is a little short (not really noticeable though) but it is a good chassis for conversion. The advantage with "Katie" is that the motor fits easliy in the boiler leaving the footplate free. The main problem with "Ben" is fixing the body to the chassis. The Hornby body is fixed by two screws - one goes through the cylinder block into the front footplate and the the other goes up into the cab.
When I built "Katie", I had to fix the body to the chassis using only one bolt. This is longer than the Hornby original and goes up through the cylinder block to a nut securely fixed in the smokebox. This conversion isn't really too difficult.
I haven't added any extra weight in the body, but the loco's dome is a rather heavy white metal casting that provides the weight.
Hope this is useful. If you want any other information about the model, please get in touch.