Thank you all for your responses, I have decided to get the cracked tank fixed (around 500 bucks including thermostat) and see what happens. I want to try my luck, I will update you guys.
my biggest question is how bad are my leak-downs. I read in many forums that leak down in and around 20% is like little worn engine but still can run. any thing beyond 30 means real problem. I want to understand from that perspective.