PS Engineering is now shipping a stand alone Marker Beacon Receiver, known as the MB-10. There are two versions, one that is panel mounted while the other is remote mounted.
Not really. Remember that the MM is not normally required, and if so, there is no requirement to increase minimums if it is inop or missing. If MM or OM is required as part of the approach and has a waypoint or NDB as part of the marker, you can substitute GPS nav to that beacon instead.
... at least I hope I have this right ... I'm off to take my instrument in a few days...