If you want use ← ↑ ↓ → and Ctrl as FIRE choose Kempston Joystick.
Keyboard: Left/up - A, S, D, F, G, Left/down - B, N, M, symbol shift, Right/up - H, J, K, L, Right/down - Z, X, C, V, caps shift, Fire - ENTER, Autofire - W (toggle), Pause - P (except in Grapple mode)