Thest might be worth a try on those bolts, hammer on an undersize one and crack them open, Mazza has had success using these on his Bognor lean to project thread over on zroadster.org.
https://www.toolstation.com/irwin-bolt-grip-nut-remover-set/p63045
Other options on ebay as well https://www.ebay.co.uk/p/23032586981?iid=124566908240