I think you may find it's a high crank battery you're looking for rather than starter motor although rebuilding will help.
Tanya batteries in Wales, haven't checked them lately but I had a battery from them called "Enduroline", it was 12 v 14ah & had 10 or 15 more cranking amps than most of the others I looked at, and it was cheap £50 ish. It spun my old Speed Triple 955i like a finger spinner.
Just had a quick look on their site, looks like they call their batteries "Powerline" now.
I would definitely buy another & will do when the 1125r needs one.
I don't work for them or am associated in any way, the batteries are really good.
https://www.tayna.co.uk/https://uk.trustpilot.com/review/www.tayna.co.ukhttps://www.vouchergains.co.uk/tayna-ba ... unt-codes/Click the tanya. co link & check sizes etc.
Trustpilot love them.
Click vouchergains and automatic discount is applied to your purchase.
Good luck & all the best. Paul