Ask HN: Strategies for porting native code to another OS?

My company has a handful of C++ server applications which run on very old versions of SunOS.

The hardware they're on is fairly ancient and we're struggling to find replacement parts (having to look on eBay...).

Management have decided it's time to get the apps onto modern h/w, and Linux.

We've identified a few "patterns" which modern gcc doesn't like, and we've got a few Perl scripts to run regexes over the codebase for those cases, but it seems like this is going to be a very manual process.

Are there any good methods/techniques/tools for porting C++ apps over to a new platform?

Would it be easier to re-write the applications in a higher-level language?