It sounds like they've done a lot for you but I may be saying something you don't like here. It's a tendency with people that support us that we feel obligated to accept things more so when they have gone out of their way for us or helped us in some ways. Some of this is natural and normal and even deserved (by the people that have supported us) but sometimes and I fear it may be the case here, we feel overly obligated to them and even afraid to say or do anything that might deem us "ungrateful". They helped you, sure and I know you actually are grateful. Hopefully they already know this but be careful of putting them on a pedestal and being worried about saying anything that might imply they dont' actually do EVERYTHING right. I say this coming from a place of having done the same things with my family in the past. Their good points should not make them immune from having faults.

Your father with good intentions has promised to do something for you but at the same time has not made it a priority of his. That kind of gives me mixed feelings about the whole deal. after 3 yrs the fix that I'm sure would take just several hours to do kind of doesn't seem very important to him.

You can be assertive (lol coming from me you should chuckle) and be respectful at the same time. Tell him the truth and ask for it back .. you've already offered to pay for the work. There is some credence to the idea of being the squeaky wheel too... just a thought.