Yeah lots of work. All you are building is an A14 or A15 engine.

Why not just put an A14 or A15 in there?

If you go the A14 crank way you might opt to machine the crank rather than the block. The con-rods will have to have the small end bored (or as qik100 said source some A12A rods) to accommodate the bigger pin of the larger A-series pistons (if you were stroking the engine as well as boring it).

There are a few ways of doing it but it's much easier to drop a larger engine in.