Most Purina milk replacers are OK; I had my last bottle calf on a purina milk, but I don't recall offhand which one it was. Is yours a 20% fat, 20% protien milk replacer?
Usually give injections in the neck. Penicillin goes SubQ, not IM. Depending on what her problem is, you might need something different than Pen. What was she on before? usually you don't repeat the same treatment if it didn't work and she's had time off of it - in theory, that creates antibiotic-resistant bacteria.
I'd try the electrolytes (homade version is fine), Vit B, Bo-Se, probiotic, see if that makes a difference. If not, there's LA200, Nuflor, etc that might be worth a try.