Ideally, you would keep your iMac and buy a PC as well. I know that might be out of financial reach, but both have their perks and flaws. One of the macs flaws of course is what you are running into now, the hardware is fairly weak when compared to a PC of the same price range.
The requirements for that particular sim seem to be quite low. I am sure you would have to turn the graphics options down, but depending on the year of your iMac you might just need to buy a retail copy of windows 10 and bootcamp it.
What year is your iMac?
(click on the top left "apple" symbol, then click "About This Mac")
e.g. iMac (21.5-inch, Mid 2012)
If you really want to buy a PC, I can help advise. What is your price range? Would you feel comfortable building your own?
If you buy the "full retail" copy of windows 10 home, you could first try it on your mac using bootcamp (free) and see how the sim runs. If it does not run to your liking, you could build a PC and use that copy of windows so you would not be out anything. If you do not plan on building your own, you will have an extra copy of windows, and be out about $120 USD.