That depends, are you asking for 500hp at the crank or at the wheels? There's usually a 20% diffrence between the two. To make 500rwhp, you're likely to need about 600hp at the crank. Note that most, if not all, automobile manufacturers list a car's performance stats in crank horsepower. 500hp at the crank would be about 420rwhp.
Anyway, many would be well to agree that a mostly stock 7M can attain 500 crank horsepower with little extras. An MLS (muilti-layered steel) headgasket (HKS is considered the best available), is moreless the main requirement for internal modification assuming the rest of the engine is healthy.
But since you have to machine the block and heads to get the HKS gasket in, you may as well shot-peen and beam polish the stock rods (or get forged rods), and do any machining that a GOOD engine builder recommends. Replace the bearings and make sure everything is in spec, new rings, new gaskets, and if you choose, new pistons. IIRC, Doward intended the Probe pistons to be used right in around 500hp.
A stock 7M head will reluctantly flow 500rwhp, but a port and polish will help the engine breathe alot better to get that kind of power. People have run that kind of power with stock valve train, but atleast replace the springs and stem seals to prevent a failure. You're not likely to enjoy your day should you break a spring at 6000rpm, interferance engine or not, lol.
A new timing belt, and fuel control and quantity to suit, and you should be ok for 500 crank horsepower. The pistons and rods aren't known to be reliable at anything more than 500hp, so you may want to consider forged internals if you ever want anything more. Keep in mind forged internals have disadvantages, there is no free lunch.
Pay to play, so go do some research... don't just take my word for it.
