Can I swap an IRF7319 Dual MOSFET for another cheaper one?
There are more options than you think: -



The dual device specified need not be a dual device and this frees up the design to find a really good P channel FET whilst not worrying too much about the performance of the N channel FET.

The three important parameters for the P ch fet are 


voltage rating (OK), 
RDS(on) (also OK) and 
Vgs(threshold) (also OK). 


The N channel can be virtually any old N channel because it doesn't get called to conduct power supply amperes.

By the way the DMC3028 looks fine (better than the original for this design)