Matt,
You will need to buy the battery and a charger capable of 8S - I was thinking more along the lines of an iCharger208B+ or iCharger 1010B+ to keep it simple, safe and automatic.
My friend Ambrose beat me to it on the AH stuff... a single 5AH 8S pack would very likely get you further and surely faster than your 24v SLA pack (which as ambrose points out, never gives what it claims... usually they are rated at 1/2 or 1/4C discharge).
I am going to step up though and say hold it a minute... take a step back and lets work out the details before you pull the trigger on any orders
Scan through the upgrades the guy DarkAngel did on that other forum... on one bike he replaced his controller with a 36v currie controller... He also changed the rear freewheel to an 11t-34t in order to be able to better pedal assist.
Now...
1.) How fast do you want to go?
2.) What kind of range are you trying to acheive? What range do you have now (or when the SLAs worked right)?
3.) What is your technical skill level? Can you use a soldering iron?
If all you want is a little bump in speed the 8S will deliver since it's resting/nominal (where it will spend most of its time, even with a 35A load present) voltage is: 29.6... I would guess it will take you to 22mph sustained and provide better throttle response than you have now but...
You could also replace the motor freewheel sprocket to increase top speed a bit and then 8S would be like a 10S (which is 36/37v nominal and 41.5v hot off the charger) in terms of top end.
No matter what you do, anything beats a 10AH SLA 24v pack - I do mean anything (think hundreds of AA soldered together)
Since there wasn't much going on here tonight (or on TV) I did all the research on your bike (actually a friend from here has one too and wants to upgrade it... so the research is general purpose) and really it seems the best path to upgrade:
1.) Replace stock controller and throttle with 36v version
2.) Upgrade to minimum 10AH 37v LiPo (6lbs total weight, 3 lbs per 5AH block)
3.) Replace rear freewheel with 11-34t
If you just want to TEST a lipo power source... I would strongly suggest you purchase a good charger like the iCharger 1010B+ or 208B+ (these could charge even a 20AH pack in 2hr / 1hr respectively) and a matching power supply and a pair of 8S packs for parallel use (10AH).
The charger can be used for any other batteries you may have or may get / build... and can perform up to 10 and 20A rates of charge.. one advantage of LiPo is 2C charge ability so even with an 8S10AH pack, you could in theory with the 208B+ charge it up in less than an hour at a 20A rate
That setup will surpass (by far) what you have now and the NiCd offering, if you use conservative charge cutoff (setup in the charger) perhaps 4.16v per cell and don't over discharge the pack (I'll explain later if needed) then you could expect perhaps 1000 cycles or even more (I have one pack here, it's ugly but then it has over 1500 cycles on it)... and no memory + 6lbs riding weight (throw the charger and supply in backpack or trunk if needed).
Feel free to post questions back to this thread or PM me directly...
Just give some thought to what you really want to acheive, this is one rabbit hole you can seriously fall into
-Mike
PS:
Gary Goodrum at tppacks.com has an 8S HVC/LVC board (active cutout when too low or high) which can be used to protect your LiPo packs... You could also use a CellLog8 from hobbyking to monitor the 8S pack and set the alarm to 3.2v which can also be wired to interrupt the throttle and cut the motor out.
If you use gary's type of HVC/LVC then you can also get a 40-50.00 24v S-350-36 power supply, tune its output voltage to 33.28v (4.16v per cell X 8 cells) and you will have a BULK CHARGER (no balance though) with approx. a 10.5A charge rate (so 1hr for two of the 8S packs in parallel which is known as 8S 2P).