The car batery is risky , it will work in an emergency but not reccomended. You can put the battery on a trickle charge to get it back to power, You waited months to ride another days not going to matter. However you should treat bike batteries as almost a disposable maint item, they only last 2-3 years then need to be replaced. Best option would just go buy a new battery, and if possible buy a small battery tender, its a really slow charge you leave on all the time, that is if you have a place to plug it in