What kind of bar stock are you using? Rectangle ? Round? It makes a difference in how you start.
If using round stock, the first thing you are going to have to do is mill a flat surface to drill the holes for the bushings and press them in. The lathe will not be much help unless you can clamp the part and use the cross feed like a milling machine with a mill bit chucked up in the lathe head.

If using rectangle stock, it's a little easier. Your first concern is centering and aligning the holes for the crank and wrist pin. Make sure your bar stock is square and has no twist, mark and drill the holes for a press fit for the bushings. Once the holes are complete, its just a matter of removing the matterial you don't need from the rod to get the shape and final dimensions you want. This would be easier with a vertical milling machine and small surface grinder. But if you're very carefull, you can do it with a drill press, milling bit, and a cross feed vise. The hardest part will be clamping your part at the angles you need and squaring the vise travel with the mill bit. Like I said earlier, I'm not a small engine expert, but I'm not a novice machinist either. Hope this helps.