Great choice, Ubi. There are reps, but they emanate from one Vietnamese cobbler in Ho Chi Minh City. They tend to have an OEM movement but they suffer from what is known as the "Stubby Dial". In otherwords, the dial is an easily spotted fakeroo. Most SM300s on eBay are fakes.
I think your best bet is to purchase a NOS Watchco kit (if there are any left).
My choice would be to have an original vintage piece because wabi/patina is of the essence for the SM300s. Just my opinion, but the SM300's true beauty comes out with 30+ years of wear and tear.
By the way, this same cobbler also reps IWC mark Xs, XIs & XIIs - they are not very good reproductions, but they remain the only reps out there. He also makes other various military watches such as the infamous Longines Greenlander and the Omega Type 53.
I have postponed purchasing an SM300 for many years and now the prices are heading towards a ridiculous benchmark. I should've I should've I should've... And Watchco is steadily running out of the world's last great horde of vintage Omega parts. Oh well, what new with the vintage watch game