I use a sensor safe RVT to seal an engine. On my TRX engines I found that sealing the backplate and carb neck are the most important parts to seal and are usually all that needs to be done.
Remove the black plate and run a thin bead of sealant where the plate will meet the block. When you put the back plate on start the four screws and lightly tighten until the sealant start to bulge then stop. Let the sealant cure for a while then come back and tighten the screws.....Do not turn the engine over until the sealant is completely cured!
Move on to the carb. Before you loosen anything make sure your carb is lined up correct then make a mark on the carb to the engine block, this is for when you re install the carb! Remove the whole carb from the engine and run a thin bead on the carb neck right under the carb body, don't get messy, keep it neat. Re install the carb making sure you line up the mark you made earlier.
In both places I just referred to you will see a factory seal, it's probable a thin orange o ring....I leave them in place and put the sealant over the o ring. After doing this I have notice a big difference in temps and tuning.
BTW...Don't seal under the head! If thats where your leak is replace the shims.