Using Li-Po or LiFePo4 for diy motorcycle battery
I would stick with the standard lead acid style of battery. They are safer, cheaper, more rugged and drop in replacement. Another real good reason is that the Lithium style batteries require very specific and stringent charging circuitry. You would not be able to simply connect the Li style batteries up to the existing charging system that was designed for a lead/acid style battery. If that was done you could risk battery explosion or fire. 

If it seems like the lead acid batteries that you are having to replace are not lasting as long as they should then I think you would do well to investigate this and make the necessary corrections. 


Does the existing charging system overcharge the battery or apply too high of voltage?
Is the voltage regulator on the system set too low such that the battery never really fully charges and dies a slow death because of this?
Is your battery usage duty cycle to charging time not optimum? (i.e. many starts with very short ride times).