:welcome:
The older Z's are quite dear to my heart, so I'd most certainly recommend upgrading such a classic. :thumbsup:
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With rechargeable cells, your best bet IMO are a pair of AW 17500 LiIions.
In theory these bodies can be bored out slightly to accept 18mm cells (i.e. 2x 18500), but for the cost and inconvenience I don't really think that the modest increase in runtime is worth it for these longer (i.e.non-18650) bodies.
I have an 18mm-bored similarly-configured older SureFire C3 (a semi-collectible 3-flats) which I now regret having had done; it would be just as happy unbored with 17mm LiIons these days.
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I will leave it to others here for dropin recommendations but my first two thoughts are the Oveready Triples or the Malkoff M91's, as these units are a near-perfect match for the higher voltages from 2x LiIion (or even reasonable compatibility with 3x CR123 lithium primaries for backup).
Most of the lower-end high-output LED modules are designed for single LiIon cells only - which aren't appropriate for these longer "3-cell" SF bodies.
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Edit: Since you mention that your 9Z still looks to be brand-new, you might want to keep in mind that these are collectible lights now.
There are of course very durable but their values have increased substantially and if it was me I would handle it somewhat carefully during use.