Welcome aboard! I see you made it over here from the Yahoo site. This site has lots of people knowledgeable in trawler systems and I am sure you will find help in replacing your rudder. I will give you my thoughts:
First, like I said on Yahoo, Mainship is no longer in business, they went bankrupt a few years ago and the company that bought them, Marlow Yachts does not support the older Mainship boats with parts.
But there are a couple of ways to get a new one. Maybe the easiest is to buy a standard rudder from a company like Buck Algonquin who makes rudders and shafts and also makes bearing and packing assemblies. Look here for a series of rudders that might work for your boat: Bronze Special Rudders On Buck Algonquin
They don't look like exact replacements for your rudder, but if the overall dimensions are ok, one of these should work. And it is entirely possible that Buck Algonquin made your rudder for Mainship to their design and maybe BA could build one for you. BA is the only supplier I know about for bronze rudders, but maybe someone else on this forum knows of others.
If you click on the small box on the right of the forgoing BA page, it gives rudder reference diagrams for their rudders and on the lower right of that page it talks about custom castings to your spec. They might still have the old Mainship molds if they made your rudder.
The stub on your broken rudder looks very small, an inch or less in diameter. The 22" long blade on BA's web page has a 1-3/4" shaft diameter for example and their smallest which has a blade length of only 14" has a shaft diameter of 1", but that one looks much too small for your boat.
BA's rudders are cast bronze which is the best material for this sort of rudder.
Another way is to get a local fabricator to make one for you. There are two possible ways:
One is to make one out of bronze plate, but finding the right material will be tough. Your fabricator will have to grind the leading edge round and taper the trailing edge. Then cut a slot for the rudder shaft and braze or maybe use a bronze shaft and weld it in place.
The other way is to make it out of fiberglass. Sailboats use these all of the time, but the smaller and higher stressed power boat props are usually bronze. You will have to start with a stainless steel shaft, weld up an "armature" to it to distribute the loads horizontally, then encase it in structural foam and fiberglass the sides.
All in all, I would buy a standard Buck Algonquin rudder even if it wasn't exactly the same shape.
Good luck, others will have more ideas for you.