We have not been able to use our outdoor heater for a couple of years. I took the cover off and cleaned with a pipe cleaner. My husband thinks I need a new thermocoupler, but I can't find one the right size. Can I order one, or can I use one that is too long from our local hardward store? Please advise. Thanks.

Try cleaning the gas tube with a pipe cleaner and insure the flame is in contact with the thermo coupling.

Clean water filter yearly or monthly. Page 16 example manual.
http://waterheatertimer.org/pdf/Bosch-125BO-NG-LP-outdoor.pdf
Shut off the installer supplied cold water supply valve.
Disconnect water pipe going to the back of the water
valve. Remove filter from back of water valve. Clean the
filter and inspect for damage. Replace if the filter is at all
damaged. Inspect the top of the water valve for signs of
a water leak or corrosion. If present, rebuild the water
valve.
Bosch service bulletins:
http://www.bosch-climate.us/bosch-hot-water/aquastar-service-bulletins/38b-125-series-bulletins.html
Descale heat exchanger yearly.
http://waterheatertimer.org/pdf/Bosch_Descaling-heat-exchanger.pdf

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

If your looking for an "off the shelf" connector, good luck. Intex uses their special fittings that fit nothing. Although what I have found works well is the adapter that comes with the skimmer. I use it for our manual vacuum and it works great.

This is the adapter: Skimmer Plate

The Polaris 65 is a pressure side cleaner. The various configurations for the wall fitting (return fitting) are at this link: http://www.polarispool.com/files/manual/PolarisManual165-65.pdf this adapter will get you to a standard 1/5" hose nipple. If you need something else you can go to Home Depot (or any other hardware store) and get a rubber coupler to adapt it to hard PVC fittings.
Something like this, but hardware store will have them in many sizes. Rubber Coupler
If the blue adapter I mentioned above works on your return(it should) then all you need is a 1.5"-1.5" rubber coupler and another piece that looks like the clean-out I linked to below. If you can find one that will fit directly into the rubber coupling, great, that's all you need. If not, you'll need a short piece of 1.5" pipe and a clean-out that slips over the pipe. The Polaris Universal Adapter will screw right into any 1.5" female threaded fitting.

1.5" Rubber Coupling

1.5" Clean-out You won't need the plug for your application.

A thermo couple is universal can get at lowes/Home depto

Try cleaning the thermo coupling, sometimes soot will cover this and it cannot detect a flame, light abrasive sandpaper will work. ( The thermo coupling is what the pilot flame touches

The outdoor fireplace should not have smoke in the loft. Have you tried cleaning the unit and making sure to remove any remnants of previous fires? The burning ashes could be causing the increase in smoke. I think that you should be able to find a solution that does not involve adding a liner, but I would try a thorough cleaning first.

The main area that you have to clean is the pipe leading from the back of your dryer to outdoors. Pull your dryer away from the wall. Unplug it. Unfasten the vent pipe. Now look into the back of the dryer exhaust and remove any lint you see there then look into the vent pipe and remove any lint. Finally go outdoors and remove any lint you see in the pipe or the pipe grill/cover.

I think you are on top of your problem,,you dont need any one elses advice.
Complete rebuild of the cooling and heating system,, simple.

the "click" you hear is the pilot valve closing. the pilot valve is spring loaded to shut off when a flame is not detected or is detected then is not. try this desatech site. it is for reddy heaters but they might have the thermocouple you are looking for.
www dot desatech dot com or
www dot robinrents dot com/heaterparts.htm