What LeslieTroyer said. A4s are pretty good engines. Figure out if your problem is fuel or spark and fix it.
Ignition: There are electronic ignition kits available. Moyer sells a complete Pertronics distributor or a module for the original Delco or Prestolite distributor. Indigo also sells a kit for the orig distributors.
Fuel: New carbs are widely available. Moyer sells em, but they're basically a tractor carb and can be found all over. If you have the original mechanical fuel pump, that can be a problem - the fix there is to replace with electric. May want to change your fuel filters too.
How to DX: Maybe list some of your symptoms here or on Moyers' forums. If it's a no-start dealie, the easy way to tell if it's fuel is starter fluid. If it starts with fluid you know ignition is good. Intermittent ignition will feel like you turned the key off. Intermittent fuel feels like a miss or a stumble. The A4 isn't much more complex than a lawnmower motor - any oldschool mechanic can fix em. Those of us that own old boats need to become pretty good at turning wrenches.
A new beta will be over $8K for just the engine, and it may be too tall. You also need to change out a bunch of fuel parts, the prop, and for the beta they want a bigger diameter exhaust line. I dunno where you can get a diesel swapped in for $8K - if so that's a screamin deal.
Easiest way to repower an A4 powered C-30 is to buy a different C-30. There's hundreds of them for sale, many under $20K. Or go ghetto and hang a 15 horse outboard on the back,