The reason why weapons are replaced in DM is because by default mutator, however the other custom mutators won't work with these maps, that is why UT classic packs were made, with enhanmcements etc...so all the weapons are replaced manually, too bad zynthetic didn't replace them when making that his U97 pack for UT.
And there is still unrealshare.singleplayer, you can add to ini default gametype as that and then you can happily play etc or even set paths outside from UT to Unreal...
But some old maps are kinda broken in UT, some zones and things function bit differently (you can notice this even in the old unchanged cybrosis, or in extremelab, these tubes you travel through they are screwed in UT), it gives you that teleport effect even in SP when you get teleported, sometimes breaking atmosphere or functionality (the PSX maps or even shrakitha part 3 with the contsant teleporting back bug), the UT DM one etc and other things...
So I prefer running Unreal for those Unreal maps and for specific UT SP maps I run UT with oldskool.