Wombles
Regarding the carby first. Can you post a couple of pics from front and side? I'm trying to remember what those old buggers ran. Somewhere, a single side-draught keeps coming to mind....
Anti-coagulant - well, flushing the system thoroughly will get rid of most rubbish and any congealed coolant in the system. You can also add a caustic solution designed to remove scale and rust throughout the system, but you might find it's so caustic it erodes rusted welsh plugs and the engine springs leaks like my mates 202 did years ago. Still, it's worth a go, because if the welsh plugs are old and rusty, which they certainly will be, they will probably let go on you at the most inconvenient time.