I'd recommend that you do careful research and evaluate the entire cost/benefit relationship before you commit to this project. Best to do your research now before you start spending money.
Repowering is a pretty significant job, and there's a lot more to it than just dropping in a new engine. In addition to the engine itself, you also need to think about all of the associated components/systems. Even if you go with a "drop in" replacement for the A4, you may need to adjust your engine beds to accommodate the new block and the drive line. You should also look at your raw water cooling circuit, exhaust system, fuel supply & return, prop shaft & coupler, transmission, prop, etc.
Take a look at this