If it's a year 2000 HP500 carb you are OK and these are bullet proof engines. The problem with the 502mag is the stock exhaust reverted water so bad either from the flange or sucked it back in because the water dump is to short into the exhaust streem. If you go with a 502 mag boat pull the exhaust and check for rust inside the exhuast manifolds and look up the engine exhaust ports at the valve stems for salt deposits and/or eroding of the stem. DO NOT PUT the stock exhaust back on...just go get some Stainless Marine mainfolds and risers..they improve performance, save a ton of weight, look better and donot revert water.
On the other hand any Formula 353-383 model with a 454 or 502 mag will be real slow. Stick with a HP500 car or efi for these models.