I cannot find any information on what those seals are made of.  They are often nitrile or neoprene, both of which B-12 is fairly compatible with.  However, I cannot be certain in your case.  With that in mind...   If the engine does not run, you may not have many options.  In that case, basically, put it in and get it right back out relatively quickly.  B-12 does not need much contact time for varnish removal, so introduce it, move the engine around a bit—shaking, using compressed air bubbles, anything for movement—and drain it after 5 or 10 minutes.  If you can see inside, that will help dictate your dwell time.  Otherwise, you may have to rely on blind faith.   However, if the engine runs—even poorly—that is ideal, especially with the varnished component in question being on bottom end.  Introduce up to 2 fl oz per quart of oil capacity into the COLD engine oil.  So if your engine holds 1.5 qts, add 2-3 fl oz of B-12 to the crankcase and start the engine.  Once the engine is up to operating temperature, the B-12 will have been driven off by the heat.  If you need a repeat treatment, wait for the oil to cool back down.  Once you’re finished, we recommend you change the oil and filter as the varnish will still be present, only temporarily dispersed in the oil.