Even if you were to get another 2000 mAh cell keep in mind that the other cells in this pack are 3 yrs old and might be much lower capacity. Without capaciting testing the others on something like an Opus you might still have some cells which are low capacity and will drag the rest of the pack down under use.
Shame you're in Canada as I've got a hoard of 18650s and a battery tab welder here
Way ahead of you A@ron. I *do* have an Opus and I'm in the process of measuring the 3 good pairs. I use a DIY adapter that allows me to measure the pair without disconnecting everything. The first pair measured 3.7 ah - down from 4.4 ah after 3 years. I scrounged some laptop batteries and came up with a few good pairs. I'll pick the pair that matches the other ones the best.
I've just finished rebuilding a second battery for this laptop. The original batteries were Sanyo from 2007!
One pair was dead, but the other 2 pairs measured 3.0 and 3.1 ah! One of the scrounged batteries had Sanyo cells from 2008 with the same capacities.
Will wonders never cease!
Ed