How can I heat my ground pool? ?

a solar blanket really heats it up. Free heating.

Depending on location, solar covers will give you up to ten degrees or so.

There are also solar heaters out there that work off of the current pool pump, that deck looks like a good place for one.  Cost effective/cost is subjective for these things. How important and such. I'm in MA and  getting older, anything under high 80's as far as water temp is too cold for my wimpy *** now.  But the solar cover and the solar heater I use, keep that temp up for most of the summer.

First rule: water and electricity don't mix well.  Second rule: don't mess with electrical hazards connected to or anywhere near a pool.

Seriously, the old heater could already be an electrocution hazard to to improper design, installation or maintenance.  Don't allow anyone near the pool (let alone into the pool) without first having a qualified "ground current" test done by an electrician with the proper tools and training.

Then talk to them (and other pool professionals) about repair or replacement alternatives that won't necessarily kill you or whomever gets into the pool for the next 30 years or so.

Put a BLACK solar blanket on it and after a couple days of sun it will be plenty warm.

BLACK absorbs the maximum amount of solar energy so accept no others.

Since pool bottoms are usually white in color as your is, they reflect nearly all solar radiation so little solar heating occurs.

If the bottom of the pool were BLACK then the sun would be able to heat the pool quite well without a cover or with a clear cover.

A black solar blanket that is in contact with the water will provide the greatest amount of BTUs of solar heat directly to the water.

While my neighbors complain about their high power bills because of their pool heat pump, mine stays nice and warm simply using my black solar blanket.

I use a solar blanket.  Much cheaper . 

you're asking the wrong sorts of people ... serious pool construction companies all know where to buy heaters that will do the job.  As for doing it cheap ... that isn't happening.  you've probably got 15,000 or more gallons of water to heat and the pool loses heat to the surrounding earth every second.  {My pool was 28,000 gallons -- if the sun didn't heat it, I sure wasn't going to.}  -- grampa  {Hint:  gas heat is likely cheapest in running costs even if you have to use propane because there is no natural gas on your street.}

That is a common problem when you have things that you can no longer take care of.  You might consider how many years you expect to live there and if it is worth the trouble.  If not, then cover it up and protect it so that if will be in the best possible condition when you sell the place.  The other option is for you to either fix it or find yourself a friend who will fix it.