I think the option that makes the most sense in the USA will be to make a custom 3S pack with the original board and three of either of these:

https://www.amazon.com/1800mAh-battery-Rechargeable-Lithium-Connector/dp/B07BTS6FVM/ref=sr_1_11

https://www.amazon.com/AKZYTUE-1800mAh-Battery-Rechargeable-Connector/dp/B07TWHHQ9Q/ref=sr_1_19

These were the only two options I could find that would fit within the (approximately) 60x54x27mm battery bay, and have (probably) a sufficient discharge rate (1A continuous 2A peak). That's why the capacity on these is so much higher than most consumer LiPos; peak discharge rate and capacity exist in balance.