Are you talking about something like this?

Then I suggest one of three for the chest piece; either part of a drum from the IG Tank accessory sprue, part of the tube carry case from the 40K Battlefield Accessories set, or failing that go and buy some plasticard tubing from a modelling shop that does railway stuff.
I think there might be such a shop near Central Station, up on Bold Street. But I should warn you that I last saw it a few years back, so it may no longer be there.
As for the head, I'm currently trying to see if the Defiler heads will fit.
I'll try to post some pics soon.
Edit: I was quick today!
AoBR Dreadnought to a Mk IV Dread.



Any questions?