A few thoughts -

A RTR with a unmarked brushed motor is probably a 550/540 silver can, or a smaller 380. These "silver can" motors don't generally draw much in the way of amps, so lower C rated packs can work for them fine.

Until the last few years, the vast majority of Lipos- regardless of how they were labeled or sold - were around 25C or less. A true 25c 5000mah pack is a pretty decent battery, good for most car applications until you get into some bigger amp hungry motors. Traxxas Id packs are not great for what they cost, but from what I can tell, (not a lot of info) they come pretty close to being a true 25c pack, so I doubt there are many brushed motors sold in RTR RC's that will stress one.