Try these addresses.
x0E933 : Tile to use for stomped Goomba.
-- Tile 38 is the unused 5up sprite which is normally used to for a squished Goomba's graphics, so change address 0x0E933 to 38.
x3F47B : Change from 10 to 30 to make Goombas die when jumped on like the classic Goombas in SMB or SMB3.
-- Self explanatory. Just change 0x3F47B to 30.
So there you go, two easy hex edits to get what you want with no patching necessary.