I've noticed the flavor to be smokey and subtle. I think paprika would work well, especially if you have smoked paprika, maybe with a bit of regular chili pepper.

I have used four parts paprika and one part cayenne with sucess

Aleppo pepper is a dried red pepper, fairly mild and not agressive. A substitute would be piment d'esplette or just a good Spanish pimenton as in Pimenton de la Vera.