I knew which banks were bad as only inputs 1 and 4 were affected (Note, the 4 banks of RAM and Acquisition chips DON'T line up with the physical order of input jacks!)
Then from what errors I had, I took some educated guesses as to what actual RAM chips needed to be replaced.
A bit of hot air and a fine soldering tip and I swapped out the chips one by one until I had no more errors or distortion of the displayed trace.
I had some salvaged boards from scrapped scopes so the chips were free, so I tested them and kept all the good ones to use which allowed me to just swap out chips freely until the scope worked.
I can also pull other chips if you need them too. I still have the scrap boards here.
It looks like the RAM chips are about $3.50 each from Digikey/Mouser, but if you want a bunch I won't ask nearly that! Postage from me to you will be pretty ok for a small parcel I think, so PM me and we can compare which way is cheaper if you want.
[EDIT]
I found my notes. The error I had was "ERROR: diagnostic test failure, dsyRastModeV0Walk, raster fail @ 1,1 read: 8, expect: f" and I've attached the display I was getting with no input.
You can see the spikes from the bad RAM. (I said things were stuck at 0, the error message has the correct details, but the RAM was bad regardless).