I went ahead and took the plunge - got a Briggs 21R807-0069-G1 from Small Engine Warehouse. Physically, it was a direct bolt-in.
Murray uses a mounting plate that bolts to the engine and frame - the nuts on the inside of the frame rail channels are difficult to get to. I didn't have any trouble...
My apologies for "necro-threading", especially with my first post - but the OP's situation was/is exactly the same as mine - Murray mower and the same model/type/code as mine.
What was the result on the OP's possible attempt to re-engine his mower? Did he go with the replacement engine model...