Moriss has an excellent point about battery choice. I too avoid wet type batteries. For many years I have been using AGM type batteries in My sportsters, Buells and Ducatis, They seem to be more compact and more CCA than a traditional but on my Ducati Monster and Buells the batteries mount laying on their sides so a regular wet type battery would not work for obvious reasons.
But on many vintage bikes I have been using emergency lighting type batteries, If you run a exposed battery like some models then you can purchase a hollowed out shell and stuff the Sealed type battery inside and you look stock but have a modern battery inside.
But the ones I use are designed for extreme service. A normal battery if allowed to discharge for extended periods the plates inside sulfate and get a coating on them that basically impedes charging or operation or in most cases kills the battery. These service type batterys can be discharged, and left like that for a year and revive just fine. (You should charge at a different rate to revive them) But the industrial applications are typically Break-away trailers where you unhook the trailer and leave the lights flashing for safety. You may come back not as soon as you expect so, while the batteries are discharged and flat by this point,, they come back to life no problem. Same with emergency barricades,,, they only get used from time to time and see a rough life, but simple to charge them up again when needed and they give great service.
For those of us who dont use these machines daily there is a LOT of appeal to that. I have a BSA 441 thats been in storage 3 years. My nephew turned the key and the lights came on shiny & Bright. (He was amazed).
Their one issue is they dont like vibration much so, I pack them in dense foam rubber to cushion them. Some their terminals are kind of small so good connections are important. There is many places selling them these days online. But back in the day I sourced mine at industrial supply stores.
Besides physical dimensions what you want to look at is Ah. Thats Amperes per Hour and that is the important rating. Try to get as many Ah as possible in the size limitations you have. Some people use these in race bikes as well with total loss systems., My friend runs BSA race bikes with these batteries (Tribsa PU Scrambler and 2 B40 units) in AHRMA race series. His motto is "When I am too old and feeble to put on my race gear, THEN I will worry about stock & Correct restorations!"
I have 2 5ah batteries down in the shop if you would like to see a picture
