Several model manufacturers have discontinued certain model kits, only to reintroduce them after 8 or 10 years of absence from the hobby market. Estes model rockets is a prime example. Almost every rocket kit has been discontinued and then rereleased after a decade or so. Several plastic model manufacturers follow this marketing plan too.
Disney corperation has been extremely successful with this marketing strategy as well. They only release their classic cartoon movies every 10 years or so, and people snap them up like crazy. Either that or they pay big bucks for them when they're out of production.
To my knowledge, no Guillow's kit has ever been rereleased.
Perhaps after the current line of Guillow's kits are all converted to laser cut, maybe they will rerelease some of the classics like the Mosquito in laser-cut format. Or at the very least, make the individual plans, plastic parts and decals available for separate purchase on the GUILLOWS MODEL PARTS LISTING.
Until that time, there's always ebay, and some Shylock looking to hawk one of these kits at a 265% outrageous markup.