Replace Old 200A Main Breaker with Newer Breaker
The MPD2200 should be correct as a replacement...

The MPD2200 should be the correct replacement main breaker for your panel -- the old Murray breaker factory changed hands a few times before ending up in its current home (with Siemens).  

However, be careful here in more ways than one

However, you'll not only need to have your utility cut the power to your house to do this safely (most utilities will not charge for this service during working hours), but you should be quite careful and gentle when inserting the new breaker into the panel, following the directions precisely.  Any discrepancy whatsoever in the fit of the breaker to the bussing and interior should not be hacked around, but be treated as time to call in the cavalry (you'll have to call in an electrician and have them call Siemens tech support for advice, I reckon).  Furthermore, you'll need a torque wrench calibrated in inch-pounds to tighten the terminal lugs on the new breaker (for the incoming wires from the meter, if nothing else) to their specified torque, as well as for any other uses the install instructions may prescribe (NEC 110.14(D) requires that the breaker lugs be torqued correctly even).

Of course, if you're not comfortable with any of this, calling in an electrician is always an option.