In the epoxy biz, the generally recommended thinner is xylol/xylene (same stuff under two different names). Acetone and lacquer thinner will also work, but the manufacturers with whom I have discussed this recommend xylene.

Properly applied epoxies will harden the wood fibers, but can also add structural integrity. Abatron in Chicago sells structural epoxies for architectural restoration of decayed and missing wood, including some load-bearing applications.