1.13 is a complete game changer that came about after Sir-Tech went bankrupt and released the JA2 source code to the general public. Everything that could has been externalized into XML files, allowing you to read, modify, remove and add content from weapon stats over shop inventories down to enemy and militia equipment if you feel so inclined.
It also fixes some features that were never properly implemented (like flame throwers or suppressive fire).
It also introduces features that range from autistic (some 20 different variants of the AK-47) over micromanaging (barrel heat for prolonged automatic fire) and busywork (food, hunger and thirst management) down to outright silly (Romero zombies). These can all be altered or outright turned off, but it requires some fiddling, reading and trying out to tune the game settings to your liking.
Most current download is always available here:
http://thepit.ja-galaxy-forum.com/index.php?t=msg&th=24648&start=0&The Bear's Pit forum is the central hub for JA2 modders, and also comes with a Discord if you prefer quick answers to quick questions.
Stracciatella is a 1.13 fork that tries to keep the improvements, but without the overwhelming complexity and with a gameplay closer to the vanilla game. Downside is that you also get much less gears and knobs to customize the game to your liking.