Castrol Purple cleaner works great. However, if I was bringing an old tank back to life, I wouldn’t worry about how to get inside to clean it out. I’d take it apart. Then it can be scrubbed.
Many of the header tanks have fatigued soldered flanges from years of thermal cycling and high pressure cycling. Plus the factory didn’t always do an admirable solder job either. Yet many have survived these stresses.
They are easy to take apart and resolder. Just use a propane torch, acid core or acid flux solder used for general plumbing. It’s a lot easer to clean everything when apart. Resolder the two pipes too.
Remember, there is a reason some cars never have problems. It’s because someone paid attention to the details to prevent a problem.