Justa thought, would a repair that did not include welding be feasible? It seems that the structure is still servicable with what cracks it has. Might it be possible that crack propagation could be simply arrested, while a replacement was obtained, rather than weld repaired? Maybe a network of bolts could be configured.

I am not of the opinion that 309 would fit on longterm. I am even afraid that this might develop fresh hotcracks.It is not expected to be a repair alloy but a fabrication alloy.
I would pepare the job by carefully gouging out the former deposit with a coated gouging rod. Do not use carbon-air rods and do not grind for avoiding excessive carbon residues. The angle should be approx 45 degrees. Then carefully fill up using a nickel-based stick-electrode like "Alloy-C" with weaving beads. Avoid high recovery electrodes due to positional welding needs.
Avoid striking- and end-craters, which may be sources for fresh cracks. Preheating is not necessary.

Before you can really evaluate whether preheat is valuable, you should know more about the makup of the cast steel. If it is plain carbon steel in the normalized condition, so tensile strength is not too high you would probably be able to get away with temporary repairs without preheat and using a temper bead techinque. If there are other alloying elements like Cr, Mn, Ni, etc, or the casting is quenched and tempered, that could make things more diffucult. However, based on the size of the casting, I doubt that it would have recieved a Q&T heat treatment. One thing that may help is to check the hardness. Is there anyway you can get a brinell reading if you can't get more data on the casting? Based on the hardness, you can determine if preheat may be necessary, and what sort of alloy may match the tensile strength if that is required. If you want higher strength than 309, you might look at NiCrMo-3. If you want a softer nickel alloy, you could try NiCrFe-2. I think 309 would typically work as well. The main reasons you want to use a nickel or austenitic stainless is for the good elongation, which reduces the stress on the cast steel part, and the fact that the austenitic microstructure will not innundate the HAZ of the casting with hydrogen that ferritic weld metal could. With the ferritic weld metal introducing hydrogen into the HAZ of the casting, it is more likely to need preheat to avoid hydrogen assisted cracking.

This is of course if welding is needed. Bonniweldor's suggestion is also good. I hear JB weld can work wonders :)

Would it be feasible to fabricate a reinforcement housing which would fit inside the area where the cracks are or maybe outside the cracked laiden area or both? Success of a repair seems pretty iffy at best and since the client is prepared order a new part (in a year) than maybe trying to weld this thing is not really feasible.
Years ago I worked at a shop where we had these vessels (cast iron) where the inside surface was required to have a specific RA finish on the inside. Because it was a casting, there were pits on the inside surface that we SMAW welded in spots and then ground down smooth to meet the clients needs using a cast iron rod. This went well, however one casting had crack in it which the client required us to fix. We used a pre heat, Vee-d out the crack, repair welded and post heated the entire casting in a coke furnace and then slow cooled in sand for 3 days but to no avail....the crack was still there.
Maintenance welding is a matter of trial and error unless you have the exact recipe for the material you are dealing with....it can be a shot in the dark with limited (if any) success.