2
There's two things you can do.
1. Find a module for 1.3.x that has the comments feature, and use that code, as the 2.0.x comments feature relies on core code, which is absent in the 1.3.x core. 1.3.x is no longer in development, so neither are the modules...
2. Simultaneously, you can make a copy of your site (on a localhost server?), and try to upgrade it to 2.0.5.1, and see for yourself if you run into many problems.
Herko