Yamaha makes a gasket sealer similar to RTV but much tougher, I used this last time I needed to open up without having time to get a replacement gasket or being careful enough to save the old one. It worked fine, still no leaks after the whole summer. RTV won't work well enough, but the Yama stuff'll do the trick.
It's called "Yamabond". I use it on crank cases only. Loctite 518 is better for side cases but still not as good as a gasket.
Talking about using sealant only, several years back I dropped my K2 Honda while away in wales several years ago and managed to break the alternator cover into three pieces. Against all odds and in the middle of nowhere, I managed to find a "nutty Proffesor" type chap who did an amazing job of welding it all back together. I replaced the cover with no gasket and about 50 miles down the road, due to the lack of clearance, the end of the crank welded itself to the inside of the cover. I ended up retrieving a used cornflake carton from a dustbin and making my own gasket which stayed on there for a few years afterwards.