You should be ok with the smaller battery.
The 998 has a shorter stroke crank, so, like the 748, its easier to turn over (less piston travel per rev) the smaller batteries are ok on 748/916/998 but 996 and 996 S / SPS can be a bugger to push over tdc with a power station hooked up to them, let alone the small battery.
You can buy the adaptor from any Duc dealer (most should keep them in stock) dont quote me but i think they're under £10. no doubt i'll be put right if they are more
