They will never balance with a different number of cells in each bank, The “short bank” will be over charged while the “full bank” will be undercharged. No easy answer to this dilemma. Cells can be replaced with that battery format but there is some removing the strap that connects the cells.
I have done it but it is not an easy task. Start with a carefully sized hole saw to cut out the connecting strap leaving a “battery post” . Connect new cell with liquid lead melted externally and poured into the gap between the battery post and the connecting strap. You will need to make some kind of “barrier” to keep the melted lead in the connection gap and not allow it to seep down to the plastic battery tops. cut pieces of insulating board is used in battery shops.
I have done time at a battery manufacturing facility so I do have experience here. Post some more details as to how this cell is damaged and I just may have a useable idea for you. Cell recovery is possible under some circumstances.