Substitute Montano, a 17-year-old who played in the club's youth and reserve ranks last term, netted at the second attempt with 17 minutes remaining and it proved to be the winner against the Blue Square Premier outfit at the New Recreation Ground.
A second-string Hammers line-up was bereft of players who have featured in summer international matches, while injuries also ruled out Valon Behrami, Scott Parker and Dean Ashton.
New signing Luis Jimenez only arrived in England this week after signing on loan from Inter Milan and will join up with Gianfranco Zola's squad on Monday ahead of their two-week training camp in Austria.
Julien Faubert, James Collins and Kieron Dyer were the only recognised first-team players in Zola's starting line-up, but the visitors nevertheless had too much quality for their non-league hosts.
Jamaican Zavon Hines put West Ham ahead on 15 minutes but Jamie Slabber, a former Spurs trainee, levelled for the hosts seven minutes later with a low shot past Peter Kurucz.
Zola made 10 changes at the break with Jack Collison and Danny Gabbidon among those coming on, the latter continuing his comeback after 18 months on the sidelines.
The West Ham squad will now head to Austria to continue their build up to the new Premier League season which gets under way with a trip to Wolves on August 15.