Radiator paint is heat resistant, but it isn't particularly wear-and-tear resistant. Agas get a lot of bashing about. And it's painted on to steel, not vitreous enamel - a very different kettle of fish when it comes to getting paint to stick.
I don't want to try and dissuade you from this, if you're determined to do it. But I still reckon you'll very much regret it.
I reckon it's worth a try if you accept you may have to strip it off every now and agin and re-do if it gets battered.
One problem will be getting a 'key' for the paint - I'd normally suggest sanding with fine wet and dry but I'd be wary of damage to the original enamel, thus de-valuing the Aga and making the process pretty irreversible.
So a very, very thorough clean and wipe with thinners is needed.
Then go to a car and motorbike accessory shop and get engine enamel. You can buy this in tins or spray - I haven't done an engine for quite a while but I used to use Humbrol. If you choose spray remember to mask stuff up, personally I'd go for brush on. It won't be possible to get an absolutely perfect finish as with stove enamel I suspect, but experiment - you can always take it off and re-do with Nitromors or other remover.
You can also buy manifold paint - if you can get a gloss that may work either. Both can withstand some heat - Agas don't get really really hot on the coloured parts.
I agree with Mr P. sounds a bad idea. But if you must then I would look at spray on stove paint. Available from stove suppliers and used for touching up woodburners, stove pipe etc. You can only get it in a limited range of colours though, many places will only stock black (matt & gloss) but you can get colours.
engine or caliper paint is an interesting idea - certainly a better range of colours, but I wonder how it will take on enamel.