Hi here I am.
I have two TEA20s, and a TR3, and have rebuilt motors on both. The heads are substantially different, though a TR head would bolt onto a TEA block, and the same head gasket will fit either. The one thing you would need to pay attention to is the water pump location on the Fergie as it changed at some point, from being on the front of the block to the front of the head.
The TR heads have bigger valves and different valve springs. The intake and exhaust ports as I recall are in slightly different locations so you would need the manifolds from the TR as well. The TRs had twin SU side draft carbs; you would want to use those of course. Redline on the TR3 is 5000 rpm, at which point your Fergie would cruise along at about 40 mph in fourth gear. Now that would be scary...
Once upon a time I did see a picture of this conversion on the web someplace, but I could never find it again. It was someone in the UK.
From the Fergie webpage you can find the Triumph one.