Let's take a minute to explore what is happening with this particular joint and see if that will provide some guidance in selecting an appropriate gasketing method:
First, this joint is is under considerable mechanical strain - the two halves of the tractor bear upon it at all times, so it needs to be physically strong and not capable of shifting once secured. That argues for a gasket that is capable of withstanding high forces without deformation, right? Hard paper, composite or similar material is generally used in this type of joint so that the gasket does not allow the two sides of the joint to move after assembly and torque of the fasteners. A soft gasket material would be problematic in this type of joint for that reason.
Second, this joint does not contain fluid under pressure. Yes, there is gear oil contained by the joint, but it is not under pressure the way that say, a hydraulic valve or engine oil would be. So, it is only necessary to seal the surfaces sufficiently to prevent the escape of relatively high viscosity gear oil at close to ambient atmospheric pressure. Therefore, the gasket material needs to be able to deform sufficiently to accommodate machining differences in the two surfaces but not much more - it doesn't need to "glue" the two surfaces, nor does it need to form an intermediary pressure-tight compound. Again, this type of joint is usually gasketed with paper or composite material. RTV silicone is not needed to effect an adequate seal.
Third, this joint will take some time to assemble as the fasteners are difficult to access. This argues for a gasket material that is stable enough to offer a long "open" time to facilitate proper assembly. Either a dry gasket or a non-hardening liquid gasket would seem to be appropriate here. Since the dry gasket is not an option due to being unable or unwilling to fully split the tractor, we're down to a non-hardening liquid gasket material.
When you look at the above, it seems to me that we're left with a non-hardening gasket material that has just enough body to fill machining differences and thus contain the gear oil, but that is also fluid enough to extrude any excess that would prevent the joint from being pulled completely tight from a mechanical standpoint so that the two halves don't shift and cause later gasket extrusion/failure.
Considering all of these factors, I'd suggest regular old Permatex liquid gasket compound - the brown gunk in a bottle that you paint on. It will seal the joint for the gear oil and it won't gum up the gears with excess. It will allow the joint to be properly torqued down to remain stable and strong. That's my view of it, anyway. Your mileage may vary, but I've put forth my reasoning so you can make your own decision.
