"....I checked the oil, which was about a quart over full...."

Head gaskets don't cause that.
Leaking needle& seat assemblies in carburetors do.

OP - ditto Bill's comments plus suggest you chk for raw fuel in crankcase oil. A simple way is to pull dipstick and smell residual oil on stick for the smell of gasoline.
If present, immediately drain/replace crankcase oil/filter and get fuel sys problem repaired before running engine again as crankcase oil diluted by raw fuel loses much of it's lubricating properties, allowing accelerated engine wear/damage.

Black smoke is from running rich. Probably from excess fuel from the carb leaking in. Never hurts to install a $7 fuel shut off valve between the gas tank and the fuel filter.

Also, if you can describe "engine sounds horrible" a little better. Is it missing, stalling, smoking, revving? Sounds like my wife when I forget her birthday. Horrible can encompass a lot of different things.

This post was edited by optsyeagle on Thu, Aug 7, 14 at 16:07

The "horrible" noise sounds like a very loud knocking

I suspect that is the Counter Balance Bearings. No practical fix if it is that and eventually it will knock a hole in the block when it breaks. Probably been run a bunch with gas in the oil. Probably why it was at a garage sale.

Walt Conner

I'd change the oil and adjust the mixture on the carb.