Engine, Loom plus Gearbox?

Nothing like this is ever 'easy', even a like for like change can cause problems.

I would give these guys a ring, they are well known and well thought of with experience of what your planning.
You might test the water with them by suggesting your looking for someone to do it for you if you supply the car, engine, etc to get idea of price, what's involved, etc.

http://www.stormdevelopments.co.uk/vw-engine-conversion.html

One thing i would seriously ask you to consider is that performance increases should be done as a package, you need to consider improving other areas such as the suspension and particulally the brakes.
More power/torque puts added stresses on other areas which were designed to work within a tollerance of the original engine.