If you have pushed your CPU to 90C you may have actually damaged it. The Q6600 was not designed to run that hot.
Also, the i3 2100 is faster than the Q6600 at equivalent clock speeds. Now, that is a Sandy Bridge CPU and that was Intel's last major performance jump. The Q6600 would be a little ahead of an i3 530 at the same clocks... but, EVERY i3 released since the 2100 is faster than that CPU (excluding mobile).
Your CPU is overheating. Dial back the clocks and see if you get improvement, or get a better cooler on it. A stock Q6600 is capable of running Fortnite, but an overheating CPU is going to tank performance and stability. If you need an upgrade look to see what the max CPU your motherboard can handle is, then chuck the motherboard, CPU, and RAM out the window because $300 will more than double your performance with something like a Ryzen 3 2200G (assuming you reuse the power supply, case, and storage). If you really can't, then don't chuck it out the window and see what the fastest Core 2 Quad you can throw in there is. You may or may not be able to find it cheaply... retro CPUs are like that for some reason. Be flexible. If the fastest isn't cheap, drop down a tier and look at the next model down.
Now, I mentioned the Ryzen 3 2200G, but even the Athlon 200GE is a 90%-ish improvement over a stock Q6600. The 2200G would be a 176% improvement in processor capability, and an 8% improvement in video performance with the integrated Vega 8 graphics. At that point you may as well just run the GTX 550 Ti though and get 4 GB of DDR4 to save some money.
So, I priced out a low cost system, assuming you can't reuse your power supply and case, but that you can use your storage.
PCPartPicker part list /
Price breakdown by merchant
CPU: AMD - Athlon 200GE 3.2 GHz Dual-Core Processor ($59.99 @ Newegg)
Motherboard: ASRock - B450 Pro4 ATX AM4 Motherboard ($74.99 @ Newegg)
Memory: Crucial - 4 GB (1 x 4 GB) DDR4-2666 Memory ($41.53 @ Newegg)
Case: Apex - SK-386 ATX Mid Tower Case w/300 W Power Supply ($43.99 @ Newegg)
Total: $220.50
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2019-02-20 14:34 EST-0500
If you can get your hands on more money you can upgrade to the 2200G for $50-ish more, and got a LOT more performance.