Storm Door Handle Installation

I am helping a friend with the installation of a storm door and handle / lock. The storm door is one that had a kd z-bar meaning that the door was not drilled for a handle / lock and thus could be used as either a left hand or right hand door. The door was installed with no problem. My friend did not like the normal handles that were available from Lowes or Home Depot. What he did want was a handle that Lowe's carries that is meant to be a replacement for a mortise lever handle / lock that many storm doors are equipped with today. However in order to install this type of handle on the door it is necessary to mortise out the side of the door frame approx. 6" x 5/8" for the handle / lock mechanism to fit into. In addition it is also necessary to mortise out the door z-bar for the deadbolt and the plunger. My question is what is the easiest way to do the mortise work? I have thought of a router but with the door assembled and installed I am not sure how to hold the router so that the mortise is accurate. I also thought about a small circular saw assuming I can purchase a metal blade for the smaller diameter. Last I thought about just using a hand or power hack saw after drilling a hole or two so that the hack saw blade wold fit into the opening. With a power hack saw I might have the same problem in not having a template but could probably use a couple of short pieces of 1 x 4 wood clamped to the door frame that would make using a power hack saw easier. I am open to any and all suggestions. Thanks in advance