If you're in the same room the whole time and check them regularly it should be fine. Cats love to be comfortable and they will move if they get too hot.
My grandma's cat used to do this. When I was a kid we went grocery shopping and came back and that cat had been fully cooked. It looked like the rotisserie we had just bought. 😢
Sorry none of that was true.
We got one of these heated mats from the pet store and our cat seems to prefer it over the space heater. It activates and warms up like an electric blanket when it senses the weight of the cat on it.
As the old saying goes, if you are cold go find the cat. She's already found the warmest spot in the house. My childhood cats used to fight over a hammock we had hanging over one of the radiators, it never ceased to amaze me how long they'd spend in that thing and the heat never bothered them. If you are concerned kitty is going to burn herself, invest in a fire guard which is a mesh barrier usually used to stop toddlers getting to the fire but is also effective for curious cats.
They almost remind me of those "greedy" slime things and also the potato thing where you put a potato in a dark labyrinth with only one correct path and it inevitably finds it and its tuber extend until its outside the box again.
Sort of like that Joan Rivers joke about having sex with her partner while they're in seperate rooms doing their own side activities.
Edit: also, vertical mazes with water being poured in
My cats both like to be near the space heater when its on. So far we haven’t had any trouble with it, but we always stay in the same room and keep an eye on them.