For transportation, most sailors use a set of beach wheels. There are many types, but here is one of the better solutions.
http://store.catsailor.com/tek9.asp?pg=products&specific=jscprreqgIf the ground is very lumpy, perhaps you can make a set of ramps to wheel your boat up again.
Putting a Hobie 16 mainsail on your Hobie 18 mast will probably not be a good solution for high-wind sailing. The mast bend characteristics is different, so you will probably end up with a very deep mainsail which is not what you want in high winds. The mast might also be at risk as it is not designed for the new load paths.
Reefing your mainsail can be a solution, but it's not something done often. Gary Friesen (who frequents this forum) have lots of experience on refing and general "survival sailing" under diverse conditions..
Depending on wind strength, a cat have several de-powering techniques that can be used. Of witch lots of downhaul and correct mast rotations are the premier ones. The wise thing to do, is to accumulate your experience on strong-wind sailing gradually. Begin with a check on your mast, be 100% sure that it is waterproof. Then rig a righting line, and practice righting your boat. Also dress according to water temperature.
There have been some nice discussion on high-wind sailing sailing on this forum earlier, doing a search might be worthwhile if you want to read about different techniques and theories.
As you live in Finland, I guess the wind is either "on" or "off" with nothing much in between?
