Yes, my 454 MAG MPI Horizon engines are still in-service on the 380DA that I have owned for 18 years.
On mine, I turn my own wrenches.
This engine has a few design deficiencies that took me a while to discover, then arrive at solutions, either by performing undocumented preventative maintenance, or by engine modifications.
Now, instead of the performance problems associated w/ those design problems, I instead have properly operating engines.
One thing that REALLY bugged me was being forced to discard an obscenely expensive exhaust system every three to four years like clockwork. I would remove the elbows for internal raw water passage inspection annually to determine if I could justify getting another year out of those parts.
At the last replacement, four years ago I had enough!
I redesigned the exhaust system, from impossible to flush and retaining raw water 24/7/365, to now draining after every usage, then flushing w/ fresh water, followed by soapy water, then Salt Away, finally draining bone dry, ready for the next usage.
My most recent disassembly / internal raw water passage inspection revealed the parts still looking brand new after four years of service. IMO, these parts should now last the life of the engine.
Some other changes I made were a custom high-volume high-pressure five-zone oil change system. It sucks my engine sump dry in ten seconds for a five minute oil change cycle. My suggestion is add this if swapping the engines. If not, mine was installed by me w/ full engine sumps w/ very little spilling.
https://live.staticflickr.com/4602/38697255400_870a0b6402_b.jpg
Another change I made was to design and implement a thermostatically-controlled heat exhaust system. When I started operating in the tropics I discovered that heat soak vapor lock would prevent restarting the engines after sitting for an hour. Now my system kicks on and off to maintain the engine room cool enough to prevent that problem. Problem solved. This was all before Mercruiser introduced that expensive kit to conceal the vapor lock by pushing fuel through to purge the vapor.
Sorry for your problems. For me the 454 MAG MPI Horizon engines are now an acceptable long-term solution. I have no idea on the 502 engines, but they are probably fine.