Rebuild or replace?

Turning a car into a hod rod usually takes more money than just fixing what you have.

So, at the very least, you can try to maintain what you have in a driveable state for as long as possible & start saving some money & locating parts.

With that in mind, one of my pals LOVES this stuff - https://www.amazon.com/dp/B00R74I5UY/?tag=joeychgo-20

He has used it on his cars without any cooling issues & is always amazed at the amount of stuff that flushes out.