Ok, so really your after parts that you may already have or could get use out of rather than a big outlay for bits.
If you have the Marine Command squad box set you probably have some great bits left over. The banner arm with the pole cut down would work for the Crozius shaft with the double headed eagle from the same kit as the weapons end (There's also the banner top from the Marine Commander plastic kit that would work). There's also some nice shoulder pads that would work as well in that kit.
For a head I'd probably try and track down a chaos player and scrounge the skull head from the Vehicle Accessories frame.
The basic marine frames have purity seals on them which would finish it off nicely.
Which isn't much help if you don't have the command squad box set and don't know any Chaos players...
Basicly take a look at the model, work out the bits that make it stand out as a Chaplin (Skull helm, Crozius) and then start having a look through spare bits you may have and ask people you know, a lot of gamers are more than happy to throw the odd spare part they may have towards a good conversion.