Have you considered just buying or trading for one?
A Strange* Market Gardener hovers around $0.60 on the Steam Community Market.
Otherwise, you can trade item for item on sites like backpack.tf - most listings for the Market Gardener ask for two weapons/1 scrap metal in return (sometimes listed as '0.11 ref').
* 'Strange' weapons track kills but are ...
No once you have got the achievement item you can't get it again. Steam support aren't going to give you another one. Crafting with Achievement items makes the items untradable aswell so its a pretty bad idea to smelt the weapons down to scrap.
There have been instances of Steam Support returning gibuses to people who accidentally deleted them. But I would bet that Steam Support has seen people try to game the system this way, so you're better off not wasting your time (or losing your hats!)
You can make an unusual weapon strange and killstreak if you have a strangifier and a killstreak kit that match the base item type.
You can't change a weapon's paint, so you need to start off with the unusual painted weapon you want.
There are quite a few cases in which it won't work though:
Not all war paints have unusual versions obtainable.
Not all war ...