It's a Perkins 4-107 internally. Pistons, rods, rings, bearings, etc. can all be purchased for a standard Perkins 4-107. The injection pump, lines, exhaust, alternator mounts and so forth are westerbeke and you don't have much choice on those.
The injectors are, thankfully, Perkins, so you can get parts directly from them.
I haven't had much luck finding cheap westie parts, either.
I've been through my Westerbeke Four One Zero Seven. There's no black magic in it. Just some expensive bits. I still haven't figured out where the gold is...I know it has to be there.