Keep your suspension characteristics in balance. Too much damping with too soft springs will cause your forks to "pump down" when things get bumpy. Too firm springs with too little damping will cause rapid rebound and make your front end bounce.
If you can afford a set of Progressive springs, installing them would be a great idea. (Under $100.) They come with a length of PVC and instructions for setting the preload by adjusting the length of the PVC spacer. You want to set the preload so that when you sit on the bike, the forks will compress about 1/2" from their fullest extension. That will be a bit on the firm side, but probably about right for a cafe racer. (I like a bike for touring to be just as firm, but that's just me.)
Next, the viscosity of the fork oil adjusts how much damping you have. 15 wt fork oil often is just about right. It's higher viscosity than generally comes from the factory, which is almost always too soft. Higher viscosity oil increases the damping to go along with the stiffer spring preload, and compensates somewhat for loss of damping caused by wear. It's not at all unusual to even use 20 wt or 30 wt fork oil in an older bike.
There is one more factor that can be adjusted somewhat with the oil. Forks are what we call "emulsion" type suspension. That is, there is no physical separation between the oil and the air in the forks. When the forks are compressed, the air space inside is compressed, effectively increasing the spring pre-load. This makes the spring action more progressive. By adding a little extra oil, (like an extra ounce,) the airspace is smaller. This, in effect, increases the "compression ratio" of the air in the forks, and the pressure increases more rapidly as the forks compress. This makes the forks more resistant to bottoming out. Too much oil, and the forks will bottom out on the oil before they run out of travel. Some will say this could cause the fork seals to blow out, but I have only ever heard of that happening once. Fork oil seals are the same type of seals used in high pressure hydraulic cylinders. Allow that almost an ounce of oil will stay in the fork when you drain them. If they are taken apart, (like for seal replacement,) they will be completely empty dry.
You've gotten several pieces of advice suggesting making modifications to the internal hardware of your forks. I don't think you should go there unless you are willing to take your forks apart several times and are prepared to buy new parts if you mess something up. You can accomplish what you are looking for simply by getting the spring preload set properly and adjusting the oil viscosity to match the stiffer spring preload. Forget about stacking coins in the forks. Why would you put quarters in there when the hardware store sells washers for a nickel? I have worked on older forks that I had to add 2" of preload the springs were so sagged out. Go to a home improvement center and buy some PVC pipe. Measure the ID of your fork tubes and get some PVC that will fit in there. Many home improvement centers have "project lengths" of pipe that are 2 feet long so that you don't have to buy a whole 10 foot piece.
