Below are my "TOP 10" upgrades:

1. Electronic Disributor. 
   
2. Electric Fan/s:    Take off the old clutch fan and install the bolts to hold on water pump pulley ( may need shorter bolts to clear waterpump). I have made a shroud out of aluminum diamond plate 12x24", and mounted 2 10" fans side by side. This system works great due to the sized of the cosmo radiator core only measures 11x22 inches. Picture of fans installed.           
   
3. Cut front coil springs:    I cut one coil off the top of the front springs on mine. Also added a bigger ADDCO swaybar and cut the link spacers down to match the lower stance. This lowered the car about 2" and greatly improved handling and steering response. Not to mention how much it improves the looks. You may want to start with cutting only a half a coil off and test the highth of your car first. If anyone know of a spring company that will make us some springs please email me. Oh and dont try to cut the rears - Rides like your on rocks and sucks on bumps - believe me.          
   
4. Pro-Car Seats.     My favorite. I ordered ProCar Rallys Velour and the ones with the universal adjusters. When I got them the factory mazda rails bolted right up! Not sure if they are still offered this way but you will not regret the superior feel of these seat. A pair run about $650.00 us dollars! You can beat that. Please get rid of those burgandy faded to green seats!                                                                                                       
   
5. Exhaust System.   I use a cast iron manifold used on the fuel injection 13b. Mine came off one of several jap import motors i use to buy. I have to weld a small opening shut to cover the stock exhaust hole. It is alot less restrictrive than the thermo reactor and quiter than a header. Plus looks stock untill smog is not needed. I also install a 3 way cat with the air injection around smog time. With the air pump on and the rear intake port connected to the air intake of the cat it cleans and passes every time. Down stream is a 30" glass pack and a 85 Rx7 muffler with custom tips.         
   
6. More Air:   Installing a aftermarket air cleaner to let more air greatly improves the performance of the Cosmo. You can use any 4 barrel designed air cleaner and use a plastic adaptor to reduce the neck size to fit on the factory carb. I am running a triangle shaped like a rotor right now. These can be found at most auto shops and here is a great site with lots of AIR CLEANERS. I also use the botton rubber gasket fron the stock air cleaner when using the adaptor. Adaptor can be got at most any parts stores. If you  cant afford or dont want an expensive new manifold and different carbuation, this is the next best thing. Pictures to follow.                                                           
   
7. Wider Tires and Rims.   Increasing the rim width to a 7" from stock 5.5 not only looks better but handles alot better when equipped with lower profile tires. I run a 14x7 custom drilled to 4x120 bolt pattern and have 215/60HR14 tires. I have ran up to 235/60HR14 tires with no problems on clearence but are sometimes hard to find. I am currently looking to upgrade to 15 or even 16" wheels once I find a style I like and a company to custom drill them.                                        
   
8. New Shocks / Struts.   I have learned that shocks and struts are near impossible to find for this car. However, I also found that rear shocks for a second gen RX7 are a great fit for the rear of the Cosmo. I have a source for the front thru Atkins Rotary. Was told he has a insert with an adaptor that will work.          
   
9. Sound Sytem. Ok this might be a givin, but the factory radio and speakers wont due. I have deleted the rear defost (never worked anyways) switch and removed the factory cubbie hole/ cassette player and cut out over to where switch was located to install my Alpine CD player with Disc changer in trunk. I have install 4" speakers in the kick panels, fit nice and recovered with carpet. Two 6 1/2 3 ways in doors (tried to hide behind panels but too big). You maybe able to get 5 1/2 for doors then hide behind recovered panels (my choice). and I have 2 8" subs in rear deck soon to add 2 6 1/2 dual cone to rear deck as well for improved rear highs. The rear deck has a new fiberboard cutout for speakers and covered with new carpet. Sounds great from the factory 3 speaker system.                                                                           
   
10. Fix High Beam Switch  I had to repair a few of my Cosmos high beam switch that would stay on. Its hard to go through the process on paper but it envolves removing and dismantiling the switch. Bending a metal tab down to make contack with the high beam clicker and reassemble. If you need help with this let me know. I can repairs yours or try and find one of my old switches and take some pics.