aviation sealant works great between puch motor halves, its what i always use (advise from the guru) and available at any auto parts store.
its basically what come between them stock, but newer and better. resists, gas, oil, grease, water, and is designed for mating 2 machined surfaces together for up to 400F. never really dries. not silicone based.
for everything else i use only gaskets or spray on copper sealant or motogrey 1