I have both a Silverston Sugo case and another micro-ATX with similar space problems (An
Antec Aria)
For the Silverstone Sugo case, I used a
Silverstone NT06 Cooler as Silverstone reccomened. I also started
this thread looking for a PSU that would pull air throught the cooler and out of the case. I could not find one (they are all thermostaticaly controlled these days). Instead I fitted a fan to the underside of the cooler so that it pushed air up throught the fins, and into the PSU and out of the case. As my system has an Intel core duo cpu, I used the fan from the stock Intel cooler. Any fan less than 20mm thick should fit, but the large 120mm fan that comes with the NT06 is to thick, instead I intend to use it as a case fan on another system.
On the Antec system the PSU does not have many vents on the underside, so I could not use the pusher arrangement above. Instead I used a
Zalman 7700ALCU Cooler that does fit in the space, though the
Zalman CNPS8000 looks to be a better performer. If your existing PSU does not have a fan or vents on it's underside, then you might consider one of those.
Bear in mind that that cooler is designed to fit into a much smaller space than most. The cost and performance needs to be judged in that light. I don't have one, but it may still represent good performance for it's size.