Welcome to the club! And get ready for an expensive hobby/pastime. Most of us here have been where you're at and there is no cheap, easy fix for repowering.
Your engines have 500 hrs, one is bad and the other probably isnt far behind.
The least expensive way (but the most time consuming) is to rebuild them yourself. You will need to find a first rate machine shop and someone experienced in building marine engines to help you along.
Option 2: Buy remanufactured longblocks or new ones if you can afford it. This is one of the quickest ways to get back on the water. This gives you the option to sell your old engines to offset some of the cost or rebuild them and have a spare set of engines. (see the link)
Option 3: Buy low hour takeouts. This is what I did. I found a pair of 496HOs (420hp) with 64 hours for 6300.00 each. This will also get you the power increase you mentioned and also have the old motors to sell/rebuild. Initial cost is higher but long term reliability and operating cost due to modern efi setup will offset that.
Once you get solid, dependable power, your outdrives will probably explode. You should have those looked at while your engines are out.