Depends on what you consider "low-cost". The first thing is an open side-plate kit for the paddle fan, to get a little smoother airflow to the cleaning system. I also have a Harvest Services throat kit and cutouts in the front of the sieve, and think it's a pretty good improvement for not many $$. Then an AFX-style kit for the rotor - I've had excellent luck with Kile Impellers. Offside concave adjusters are a good thing to add, you can fine-tune your concaves with 'em. If you're doing barley-only or easy threshing crops-only, I've talked to several people that notched out the tunnel and put in 1680 concaves - I'm scared to, we cut a lot of wheat and I'm a little short on HP as it is..... I've also talked to people that swiped an aftercooler off a newer 466 and turned up the pump, I'm on the lookout for one of those. Unloading auger extensions are an absolute for a 30' header, at least if you've run a 1680 before (you get spoiled the first time you do!) Another quick/dirty/cheap thing that works really slick with a sickle header is to move the reel height switch up onto the hydro handle - I mounted mine on the front of the shaft, so the toggle is between my middle fingers when my hand is on the grip. Oh, and a mirror the size of a pickup camper-mirror mounted about a foot in front of the upper-right corner of the grain tank lets you look back towards the cab, down the inside of the front grain tank tip-top - cheaper than a switch and warning light, and works nicer, too!
Oh, and you might want to think about reinforcing your final drives if you've still got the tin covers on 'em.