Sculptor 7 -

If your chain guard ("shield") is so buggered up as it cannot be straightened, it's time for a new crank. There is the added possibility that the rings themselves are not true either, based on your description.
You can get a truly fine cottered crank set from eBay for about $30 (one and one-half times the price of your bike!)

Just make sure you have the right size new cotters on hand because there are British, French and Italian sizes.
(I assume you already have some experience and tools for removing and refitting these, if you have already overhauled the bike.)