I want to echo Chuck's response except to add that it's not impossible but very difficult to replace the bad cell. Among the many problems are those that Chuck mentioned in that the ultrasonic welding is so perfect in achieving what appears to be a single piece of plastic out of two halves.
Maybe try:
1) carefully cut the case in two with great care so as to keep an even bonding line when the time comes to rejoining them
2) determine which cell is bad and remove it. To get a replacement that works satisfactorily one needs to determine cell characteristics such as capacity and a dozen other cell characteristics that went into the OEM cell selection.
3) spot weld the cell into the series string being careful not to overheat the cell case. This requires a reasonably good cell spot welder and considerable practice. The new connector ribbon must be long enough but not too long or you can't reassemble the cell stack so the plastic case will properly fit and can be welded together. Test the pack prior to re-assembly.
4) Re-bond the two case halves together using plastic welding, super glue, or any of a number of bonding techniques that create a thin bond line.
5) After you did all that and the battery case won't fit the original FT60, go out and buy a new battery pack and enjoy your radio.
They are really one on the best radios around and it's hard to improve on Yaesu's original design or construction. How do I know all this? I did this as an exercise to assess how much trouble it would be and found it was really a lot of trouble getting everything just right. It's really a kind of fool's errand and, to me, the cost far exceeds the benefit.
Will
Will W6SWF