Hi, I've decompiled apps before. Like whatsapp for example to find out how they send and receive messages. With that information I've built a desktop whatsapp for myself.
Recompiling and signing (that's very important) is the easy part.
Of course not any changes are possible and depending on if the creator of the app used obfuscation it will be more difficult. So to give you an accurate quote I'd need to see the apk and know what changes you want to make exactly.
Anyone, who just tells you "yes, I can do it" has no idea about decompiling.
I'm looking forward to hearing from you.