Chief Aircraft (https://www.chiefaircraft.com/radio-...s-glow-engines) carries OS 2 stroke engines in .35AX, .46AX, .55AX, .65AX, .75AX, .90AX and 1.20AX sizes. They also carry Saito 4-cycle glow engines in a variety of sizes. The Saito FA-62B would compare well to a typical .40-.46 two stroke glow engine, but would be a little heavier. However, it would drink less fuel. Still lots of good used engines available for near dirt at RC flea markets. Check your local model clubs. I bet somebody will probably still have some .46 size engines in a drawer they would be willing to sell cheap. Old does not necessarily mean its junk. Yesterday, I flew my Goldberg Cub powered with an Enya .53-4C that has been on it since 1995 and it still starts and runs like new. My Enya R120-4C has been on my Phaeton 90 biplane since 1988 and it too still runs like new 35 years later. If you can find a used, but not abused, Enya .40-45 .50CX in decent condition, it should last a really long time and become one of your favorite engines. I think Enya engines are still being made, but you will have to order them from Japan (https://www.enya-engine.com/ListAP2CYL_E.html). They show a 50CX twin needle valve model rated at 1.7 HP that should start your tractor.