I know how to do it in Fallout 2 but I can't seem to find the same file for Fallout 1
rnddesrt.int
I think this should be addressed by the makes of the mod for the reason I posted above.
Mantis' appearance in that region of the world was extremely rare since the first release of the game. So it's never been assured that you'll get the encounter (and corresponding karma levels) before visiting Shady Sands. It's more likely to find mantis near the Boneyard or during caravan run than in the area between V13 and V15. Hint: killing ANY 'evil' creatures rises your karma.
We'll check the desert encounter script bundled with the patch. But if there is nothing wrong with it then we definitely won't tincker hardcoded into the engine tables just to fulfil personal customs. When it comes to cheating, you're on your own, sorry.
--- Update ---
Alright. I've looked into the script. And it looks this way. There is only one opportunity to encounter mantis in the topmost desert area of the game world. It's so called 'Dead traveler' encounter. It could appear only in the approx one square radius around Shady Sands. And the chance is only 16,67%. It's already doesn't look like 'quite often'.

Moreover. The 'Dead traveler' in fact is not just a generic dead body but Lance, which was reenabled by Avega. So you never get the
dead Lance encounter until he actually dies. And at the moment, when player usually roams between V13, SS and V15, Lance is apparently alive. So no manti could be encountered at the early stage of the game.
You can just delete the file rnddesrt.int to revert to the old behavior. But this could trigger back party member multiplication bug and something about Tandi (I don't remember what exactly).
I'm adding this one to the list of questionable features. But I won't guarantee that anything will be actually changed.