If the 120gr does shoot well from your gun, It'll likely do good on the deer. I've shot many,many deer with the .257Roberts with a 120gr bullet at around 2,700fps.
I've only taken one doe with my 7/30. It was out my kitchen window at 140yds (lased w/range-finder) while it was browsing under a water oak tree. The Speer 115gr HP hit it just off center of the spine as it had its head down and was facing me at a slight angle. The bullet transited the chest and lodged in the paunch (intestines/stomach), hence I didn't go "digging" looking for the bullet.
Don't overlook the 130gr Speer FN. It is designed for the 7/30 velocities and at under 150yds will perform splendidly. Likewise the discontinued 139gr FN. A friend that passed away in 1998 had a Winchester M94 that came with a 24"bbl shortened to 17.5" by Walker Arms in Selma, Al. After that rifle came back, it was an unbelievable tack-driver. It too wore a low powere scope. A Bushnell Banner 1.5-4.5 if I remember correctly. He had over 100 guns at his death, but 2mos from his passing (cancer), he told me that the little Winchester was his all-time favorite. He used mostly the 120gr Nosler FN, but he said the 139 Hornady with a heavy load of H414 was the better load. It chrono'd at about 2,550fps from his shortened barrel. He said it always exited and left a better blood trail,,, IF, the animal managed to get up and run after being hit with that load......
It's too bad that Marlin has never embraced the 7/30 and chambered it in the M336.... It truly is a better cartridge than the .30/30.