that measurement you took ian is a rough idea , it looks like you have some wide wheels on there , but you could be looking for a 200mm shell, ideally take it to teh model shop and ask to try the shells, by far the easiest way to do it .
since you already have a shell you could measure the width of that one and that would give you an idea.
the chassis looks in good nick, the engine upgrade was obviously a needed change, as the oild engine could be a pig to get running nicely.
new cars are more efficient in design, driveline,chassis etc etc because they are used for track racing, the chassis you have was a semi off road /on road set up , i think the same or very similiar chassis was used for a 4wd buggy but i would need to double check on that.
good luck anyway and let us knwo how you get on
Adz..