So i just purchased a house and it came with a 1998 (im kinda guessing) Jacuzzi Premium tub. I havent verified what temperature the water actually gets to but i had it up to the 104 limit and it had been heating for at least 2 hours with the cover on. The water did not feel like 104 at all, i know that 100 is pushin it for me and this water felt more like 90.

So my question is are heat pumps a serviceable item or does it make more sense to just replace the unit.

Or...maybe 2 hours isnt enough time to heat it up from a claimed 100 to 104. I had the tub running on economy mode all day which keeps the temperature within 20degrees F within the set temperature. I got home and it claimed it was 100 which im sure it wasnt. So i switched it from economy mode to standard mode and upped the temp to 104. couple hours later it felt a tad warmer but not much.

is it possible that the onboard thermometer is malfunctioning which is limiting the heat coming into the tub?

not likely, but not impossible for the temp reading to be that wrong - usually a sensor error or OH error would occur but i have seen one instance with a faulty pcb reading the good sensors incorrectly.
however, in the first instance cross check your spa touchpad temp reading against a known good thermometer, then go from there