Unless you're a very experienced DIY'er...
..replacing the timing chain is NOT an easy task. It is highly unlikely that you need to replace your chain; many techs on this board have stated they have never seen an M103 chain break, and when they did it was due to castostrophic failure of the chain guide itself.
I DO recommend replacing the chain tensioner guide and the chain rail if it has never been done before.
The sealant you need is Loctite 5900. I don't have a p/n but phone Phil at Fastlane and he'll fix you up.
'91 300E, 213K miles