Bone stock 1999 502 mag MPI. mefi 3. Just had it totally redone and would like to add a closed cool system.
Understand it might be running a bit warmer than the 160 it runs now. Would that be enough to burn it down ?
Any suggestions who has a decent system ?

I've got the stock (as then available) Merc FWC system on a 99-2000 502 and it works great. It seems to be able to keep the engine at 170 even when the water temps are 85 and air temps are 87-90f
A lot of guys here have bought aftermarket FWC systems and had great success with them too , I'm sure they can offer lots of alternates . Not sure what you mean when you ask if it's enough to 'burn it down' though.

Try Mr Cool to see what they have.

Fresh Water Cooling Kits

many oem stock closed cooled models run at 170 to 175 degrees as they have a 170 degree T stat installed. Many current builds require heat for ---for part expansion because thats how as they were designed / engineered.. Running some of these current engines do not do well running colder than they were designed for.

99 percent of all OEM stock gas marine engines since late 1996 have a 160 degree T stat installed from the factory for inboards and I/O s. -- EVEN carburetor engines have a 160 T stat installed since late 1996.

Iin the past 8 years or so alot of closed cooled gas marine engines have a 170 T stat.

The other thing a hotter engine to a certain point lesson emissions and this all has to be met for the EPA and CARB. Start up emissions is another big concern with all motorized vehicles ( EPA and CARB requirements ) . If you notice HOW FAST autos warm up today compared to even just 10 years ago its days and nights difference because of a couple of reasons. Emissions is one of those reason, 2 is get the oil warmed up as fast as possible for better flow thru out the engine and 3 is internal part expansions for a jist of it.

Merc says there are 3 left in existence and they're $2500 a pop and to stay away from aftermarket crap, of course LOL
Who's had good luck with what ?

If I told you of my experiences with Merc at the very highest executive level you'd never buy any product of theirs again , ever !

I bought a Mr Cool oil cooler for my 496. It wasn't the same quality as the OEM. I sent it back and bought the factory replacement.

That is correct ^^^^^

Just had one in for winterization and the oil cooler was brand new installed this year. The boat owner said his brother bought the cooler and installed it in May 2018. Anyways I got the boat in for winterization, I removed the front and back hoses to drain it completely, to my surprise the backside of the cooler for the hose connection was collapsed from installing the hose clamp tight.

I could not believe a person was strong enough to collapse it from the hose clamp install. With that said the real story here is how cheap the cooler was made, not how strong the guy was.. Again the cooler was new and I am still waiting on where and who's cooler was bought. . Again I do NOT know who's cooler it was. . Sometimes you have too watch out what you buy. . Just saying.