I'm in a similar situation (small company, less than 50 employees, 4 engineers on the core system), but I feel fairly comfortable tackling it as I've done similar work in the past - basically rewriting from within. For me, key is dont throw it all away and try and start from scratch. Do the rewrite from within. Identify problem points and tackle them one at a time. Try not to get too ambitious. Try and keep the scope small. Doesnt always work, though. Thought I started a small scoped change a few months ago, ended up with a +600 file commit, the code was that tightly coupled.