Hi Mick, welcome to the site. Unless you're entirely confident in tackling some kit bashing, I'd suggest avoiding it for the time being. Sometimes taking on projects like that can end up costing more in the long run.
The ex BR generator van by Silverfox, which Rob has suggested above, is a decent enough model. It can be a little pricey depending on who or where you get it from, so look around before buying. The detail is no way near what you'll get on one of Murphy's Cravens, but it does the job, and looks well behind an 071.