With 18650 cells, they come in a range of mAh capacities... you could get better cells with more capacity from a reputable vendor and increase your playtime. Don't trust what an eBay or AliExpress vendor SAYS their cells are rated at.
A high capacity 18650 will give you no more than about 3500 mAh, regardless of what the vendor claims. They will cost ABOUT US$7 each new, and will cost you another $10-$20 to ship, as lithium batteries have to ship ground and with special packaging.
You'll also want "paired" cells, if possible... cells that have nearly identical verified mAh capacity. You'll want to charge each one up to full in the G-Boy (separately), and then put them in together so that they charge/discharge at the same rate.
That being said, I frequently harvest old 18650 cells from "dead" laptop, power tool, or other sources. I carefully remove the solder tabs and smooth down the ragged edges from spot welds. I charge them individually to make sure they get up to full, usually about 4v, otherwise I recycle them. I let the good ones sit for about 2 weeks and check their voltage again. If it's below 4v, I send them to a recycling center. If they've retained their voltage, I keep them and perform another full individual charge, and capacity test to get the TRUE mAh capacity.
Once I know that TRUE capacity, I carefully remove the stock shrink tubing (keep the white "donut" for the positive side) and then put on a new shrink tube that helps me categorize capacities (<1000 mAh, 1000-1400, 1400-1800, 1800-2200, 2200-2600, 2600-3000, >3000) and write the capacity in Sharpie on the outside.
Hope this helps.