Yep V4 engine.
My first Saturday job was as 'van boy' delivering funiture from a Luton van version of this...
..it had a V4 engine in it, like this...
There is some technical detail
hereI'd say that the hardest bit to fitting one would be to find one :D ....have you thought about older Vauxhall engines. There are plenty of 1.8 and 2.0 RWD Vauxhall cars....Mantas etc.