I have a 1983 natural gas Rheem CHA MDL# RGLA-125A-GR serial # CN4d102 F3884 9382.
Started end of last winter the flame out / heat sensor would trip. I had a new blower motor put in for AC in the summer and I have not had an issue with the sensor all this winter, it seemed to pull all the heat out and not trip. Until a couple of days ago the sensor started tripping and I reset it 3 times and it would work, now the sensor is not tripped but the unit will not ignite at all.
I was told it was probably a crack in the heat exchanger but they didn’t bother to show me, just wanted to sell me a new one (I know mine is OLD), the problem is I am out of work right now and have very limited funds to fix this problem.
Are there any other parts, sensors for this unit that can be replaced that would fix the issue described above?
Thank you in advance for your help and advice!

While the sensor may not appear "tripped", it may be worn internally. It is a safety device and not intended or designed for repeated action. Heat exchanger failures are the most likely cause of this, but visual confirmation is difficult due to the design contours of the heat exchanger. Cracks and holes can appear in obscure locations.

A 1983 model os somewhat past its prime so the heat exchanger is a good possibility. You can call a different company to get a second opinion.