If pipes slide into the exhaust port, then you have the later 1931-34 cylinders or 1934-39 Sport Scout Y cylinders. What I have heard working is plain clear bathroom silicone, it doesn't burn or melt.

On the 101 the pipes seal with cones on the exhaust spigot and in the nuts. For others I explain what I have done with my 101 downpipes..

I haven't used any sealant but put down some time to adjust the tube cones to the cylinder, carefully hammer and some carborundum. Exact tight fit like valve seat fit isn't needed at all. The nuts are supposed to clamp the cones to fit, but the tube material is thicker and sturdier on the repro downpipes, and I can't rely that the cylinder threads can take the nut force that would require, so I took the time to hammer them to fit. 

The pipes need some slack to be able to slide into the collector and turned into place, and must not be clamped to the collector solid, the pipes have to be able to move because of vibrations and heat expansion. I haven't cared to use any sealing compound but if the gap is too big it might be possible to use a small exhaust pipe expander. Or bathroom silicone.