On modern Shimano road cranks, you'll have issues finding just the one 50 ring at a economical price. At least I think you will have a hard time. Even getting the common 50/34 large and small combo together won't be inexpensive so maybe just get a 50/34 crankset. If that 50/34 crankset is Shimano and the 36 ring of the old fits onto the 50 ring of the new, then try that combo and see if you like it.

For the 2 tooth difference in size, you may or may not see any difference in your shifting performance requiring a shorter chain. That should be an easy fix anyway that you can do once you find out if you have real issues.

Just because you are in flat land, doesn't mean you should be in the big front and any of the 3 smallest cogs for your normal cruising gear. Those are for when you are pushing hard to go fast. Or at least they are for me. I also have weak legs and use a 52/36. But still I miss my 53 ring sometimes in my rolling terrain.