I'm puzzled as to how fwd tintop BECs fit everything in the engine bay. Is there a commonly used diff for these sort of things?

I assume it's exactly the same as the rear/engined RWD ones???

Engine with sprocket to a chain diff - Lots of diffs can be used, Fiesta with LSD used to be the main one, but I think people were realising LSD wasn't always the simple way forward for a lighter car - might be different in a FWD though!

Check out Cloudy's Warner R4, Craig's Mongoose and a few others...

If you are building a FWD BEC Mini the obvious choice is a Mini based diff (Quaife, Salisbury etc).
Now I wonder who might be able to supply a chain drive version of those diffs..................

Here's some pictures of an R1 in a mini front subframe.
Chain drive diff.
http://www.pro-motive.co.uk/R1KitInfo.html

and a diy one on a blog
http://fwdr1mini.blogspot.com/
and a flikr site full of photos
http://www.flickr.com/photos/22673337@N06/sets/72157603671414132/

If I were building one I'd talk to this guy about the diff ( )
http://www.westgarage.co.uk/services/chain/chaindrives.html

Ah, I see.....that's actually much more straightforward that I figured it would be!