This approach worked really well for me and is free.
1) download the beamsonic impulse responses here:
http://noox.sitesled.com/
Use the Studor +3 or +6 impulse with or without noise reduction in space designer (Load IR and init etc). Set to 100% wet. You will have to shorten the IR because it's very long. You can do that right inside space designer if you don't want to chop the wave file itself. This will give you the the tonal character and smearing of transients associated with tape.
2) Apply compression using Logic's comp since real tape automatically applies compression. I don't know the approximate ratio and threshold but you could look that up online. I would have to guess that the attack time would be very fast and the release time would be around 60 msec. I think Logic's compressor has a preset for tape compression.
Try the space designer and the compressor in different orders to see which way you like better.