Sure send it, just don’t start a forced draft program to duplicate it at any cost. The possibilities will inspire people, but you need to allow things to develop at their own pace and in their own way. The journey is more important than the destination, and frankly it will not take that much longer.
You are not converting or upgrading these weapons to 9x19mm, they already are. But duplicating the Sten or Uzi in bulk are well within their capabilities, the MP5N is much more exacting. The MP5N is a better weapon, but the Sten or Uzi are good enough weapons that are much easier to build in bulk, and you need a lot of them quickly if you are going to impact the jungle fighting. The British army did not issue submachine guns, i.e. the Sten gun, until later in the war.
Historically they did not manage to get many prisoners until very late in the war. Usually only when you found a Japanese soldier unconscious and captured him. So it will not be much of a problem.
There will first be a series of skirmishes where they try to get a feel for your capabilities, which you want to keep concealed as much as practical. Then they will send a task force to take you out, probably built around a couple battleships because they will know by that time that aircraft cannot survive near you, and their spies (and there were many in Singapore) will have told them that your vessels are basically destroyers (i.e. no armor). Hopefully they will not reason it out is that armor is not particularly effective against the weapons you have not shown them yet. You need to convince them that the only reason you did not use those weapons earlier was to lure him out so you could slaughter them, and hope that they remain unconvinced that you are out of ammo and are instead that the only reason you don’t continue using them afterwards is to lure them out again. That is why you want that task force to just disappear.
The problem is given your limited ammunition you simply have no alternative to bluffing them.