Brute forcing 50 bits of entropy would cost $1000 of AWS CPU time and a Bitcoin private key has 256 bits of entropy, so that would be roughly a 60-digit number times $1000 -- completely impossible and not even close to being profitable.
A very interesting discussion about this was done here,
https://bitcointalk.org/index.php?topic=139735.0 - Look at the explanation provided by DannyHamilton.
So you would just be wasting processing power and money that could have been used for mining and your returns on that might have been much better in the long run, if you stuck to Alt coin mining. ^smile^
If you look only at the P2PK utxos (not at the P2PKH ones), the problem would boil down to getting a private key from the public one. No hash256 or ripemd160, no addresses, only public keys.
The difficulty of the discrete logarithm problem is "only" 128 bits, not 256.