I've brought a Faster 2 Style second-hand (in mint and white), but the changing bag has seen better days. Is there any way I could get a new one? And a new pram mattress, too?
There's no offer of only these, usually included, parts in shops. Also, if it's a color that is no longer available, the chance is even slimmer. I will suggest first contact any distributor of this brand around you, or the manufacturer, the Baby Merc brand, directly, asking for these accessories as spare parts. They might but might not help you (it's not really a huge chance but if you really need it in the matching color, it might be worth it).
My other advice would be to just get a universal nappy bag - anything ranging from well-known Babymoov, Skip Hop, Childhome, Pink Lining, and many others would absolutely fit and make your pram look more different, more joyful.
The mattress should not be too hard, there are stroller mattresses widely available even from universal brands. Just visit a baby gear-oriented shop/retail and ask for advice as to what is available. Usually, you can try it out (if it fits). Also, if needing the original one, try writing to a distributor or the manufacturer about the mattress as a spare part... but it's not that high quality to really need the same brand mattress, to be honest. It's a cheaper one, price- as well as quality-wise.