straight swap yes.
may as well get the top engine mount from the valver, and the box with its mount too. the boxes are interchangeable, its just up to you if you want longer ratios.
literally drop it in with the 1.8 engine loom, plug the 1.8 ecu in and turn the key (Ideally you need a Ph2 valver engine loom) This way you may get lucky and the interior loom should match, if not you'll need to do a bit of swapping of wires on the connector block, not a hard job at all. or swap the interior loom over which isnt too much hassle.
The valver engines go for very cheap money, I couldn't shift mine and gave them away for free with mounts/looms, as they were just taking up space. Ive seen quite a few others give them away too. The JB3 valver boxes (circlip diff models) go for cheap, ive seen them for as little as £20.
I have a Ph2 ECU somewhere, and a random assortment of Mk1 engine looms lying around. Saying that I have most things apart from any valver bottom ends, got manifolds and cylinder heads collecting dust everywhere.
All in all a simple swap, the engine bay chassis on all Mk1s is the same.