OK, it sounds like any remote which can send RC5 should work-- but you probably want to know what device and command numbers it is sending. There is (or was) some standardization of RC5 device and command numbers. One good approach is to get a universal that claims to support Marantz receivers, because Marantz publishes the IR codes for each function in their receivers. So you can work backward from Marantz' documentation to figure out the device and command numbers that the remote sends.
I think the safest thing is to get a OneForAll (or sister brand) remote, because the signals these remotes send are tabulated. Best case is to get a OneForAll that has either the RC5/RC5x or RC5/RC6 Combo protocols. You can program each button on such a remote to send any RC5 device and command number.
Use the
Lookup Tool, Protocol Search, RC5/RC5x or RC5/RC6 Combo protocol. Leave "%" signs for the devices and click Search. On the next page, look at the far right column. Click on the number and you'll see a list of possbile remotes. Anything starting with URC is a OneForAll, 15-xxxx is Radio Shack, and most of the others are cable company remotes.
For example, choose RC5/RC5x Combo, and the first entry has the number 53 in the far right column. That means 53 remotes have that setup code, and are known to be suitable for your task.
If you do buy one of these remotes, we'll tell you how to assign EFCs to each button for the particular device and command numbers.