if it's a cassette rear hub, you won't need a new wheel. it is not much work at all, just take off the old derailleurs, shifters, cables. Then, you can shorten the chain, take one of your front chainrings off. If you have a cassette rear hub, you can reuse one of the cogs and put it back together with spacers. If not, you can attempt to thread a singlespeed (bmx) freewheel onto the hub, but your chainline may be off. If you don't mind dropping a chain here and there, not that big of a deal. Sometimes you can use a multi-speed chain with poor chainline because they are able to run at some extreme angles, but you may still drop the chain. If you want to be truly ghetto, you can simply use your freewheel as is, and just run the chain around one of the cogs of the cluster, but the look is not the best.