First comment is that it will be kind of hard to find a use Tiger spinnaker in excellent shape. There are not enough of them around in the US. I have a Tiger and I would love to find an extra working spinnaker just for day sailing, so I could save my racing spinnaker. New ones run about $700.
Second is that I think you are shortchanging the Miracle with a Tiger Spinnaker. I think you should look at something like an I-20 spinnaker. Take the measurements from the I-20 and copy them exactly. You will probably have better luck finding a used spinnaker from that boat as there are more used ones around.
Third comment is why are you doing this at all? For the price of putting a spinnaker on a 20 you should consider selling the boat and putting the money into buying a Tiger. It may not be a wash cost-wise but you will have a boat that you can get some money out of when you decide to sell. You will also be sailing with boats that are similar (other formula 18s). There are not many Hobie 20s that sail with spinnakers, and even fewer race.
Another thought is that a 20 will have a few problems with a spinnaker. There is a big overlapping jib that sheets to the trampoline about the same place you will be crossing spinnaker sheets. I feel there are too many lines on the Tiger and it even takes advantage of a self tacking jib. Another problem is the "Comp-Tip". It is not designed to take a load like this and you have to run the spinnaker above the tang on the mast to keep clear of the mess there. A spinnker is very delicate and does not like anything to grab it weird. I know that the mast will take it for a while as I have seen some 20's fly them, but for how long?
By the time you get all this set up you could be about $2000 into the experiment. There are sheets, sheet blocks, halyard, blocks for that, snuffer, spinnaker pole, hardware on that, how to mount to crossbar, suspending pole under bridle and wires to stablize pole. Take a close look at all that is involve on a spinnaker cat then decide, will you use it to race or just as fun factor? I think you should consider a boat that is set-up and designed for spinnaker fun.