It's been a while since I read Marx and Engels, but I'm not sure they would have been speaking in terms of conversion by example. IIRC, they thought of communism as a more-or-less inevitable development from capitalism, and that it would develop somewhat orthogonally to nation-state boundaries but establish itself first in those nations that were most industrialized (and therefore had progressed the furthest in Marx's future-historical timeline). At the time they were writing, that would probably have meant Britain.

The idea of socialism in one country was a development of the Russian Revolution, and is something of a departure from Marxism as originally formulated.