The weight saving vs the convenience... I`ll go the lecky start any day.
If you remove the kickstarter and gear and bits and pieces, the battery box and the lead acid battery, and use the a123 battery there is only a small weight disadvantage, maybe 300grms difference I can`t remember exactly.
The batteries in the photo are joined with copper strips with are soldered directly to the batteries.
I have a Mk 2 version where they are not soldered so that the individual batteries can easily be replaced, if necessary.
I made the second battery just in case the first one fails, as it is unique and I
can not just go out and buy a replacement.
So far the first battery has proven very reliable and have not used the second.
I have a special charger and
top the battery up from time to time, takes about ten minutes.
Being only 4 cells means the bike needs to be in good starting order
as it will understandably not keep cranking it over for ever.