Stock bottom end will mostly be safe for around 220bhp, some have ran more and been ok but others have ran less and had rods snap, so it's up to you really.
A set of forged pistons and rods would set you back around £800 if you wanted to be 100% safe.
In addition to the above, the old MFI fuelling system can be very unreliable now it's 20+ years old so a EFi conversion would be very worthwhile if your going to the effort of re-shelling and rebuilding the engine, there is a guide at the top of this room for the cheapest way to go EFI