I happen to have old 32-bit laptop and 64-bit desktop so I could cover both, for now. I could try, but I can't guarantee the results.
"Try, if you like" is the idea. Our build system is much simpler to contribute to than what you report the dpkg process was back then. A single, simple directory tree -- mainly just the single .SlackBuild -- builds both the 32- and 64-bit versions. If it's run on 32-bit, it generates the 32-bit package; and the same for 64-bit.
Our build system reads the files, sets itself up as a 32-bit machine and does a build, then sets up as a 64-bit machine and builds again. It's not full packages that are submitted, rather just the directory contents: icewm/src/* in this case.
Take a look at https://bitbucket.org/VLCore/vl71/src/HEAD?at=master
. Click through to var/vabs/icewm. You can copy that then edit what's there, or you can start fresh with sbbuilder, issuing 'sbbuilder --manual | less' will display instructions. Or if you haven't explicitly removed the original icewm-1.3.8 package, the SlackBuild and the patches/ directory for building are in your own /usr/doc/icewm-1.3.8/ directory. As far as patches, recall I had trouble with those. Some, if not all, have already been applied to the newer source.
Test building on both architectures is the far better idea generally, but if that's a lot more work for you then one test will probably do in this case. If you find you completed something you'd like to submit, there's a recommended way to go about that and also another way sometimes used when the first is inconvenient. Just ask if and when you're ready.
As with most packages, a fairly plain, fairly vanilla build of icewm is what's wanted. If that's the result, then any conflicts with icewmvmods or maybe VL_Theme-Light should properly be handled by changes made by someone else in those two. They're set up to work with a "regular" icewm package, and looks to me like that part was done right. Conflicts with those aren't expected and shouldn't concern you.