Hi mate, 90psi is terrible so a rebuild is a good idea!

You only need pistons if your going oversize (this will depend on the bore wear) and you do not need conrods.
You will need a full gasket set, full bearing/shell set, rings, and i'd get a new cam/lifters, water pump, and oil pump too.
All these things can be bought from Burton Power so go to their website and price it all up mate although some things can be found cheaper else where but it's a good place to start, if you dont need pistons then i'd say around £450 for the rebuild if just replacing like for like and not upgrading things.
Retarded timing on boost is either Knock sensor (wraped in rag out the way is the best place for it!), ECU, or ACT sensor.