Firstly, I'd be tempted to get rid of that pogo and patch the mod so the player starts with it. This mod was created well before that patch, and it would probably have been used if it existed at the time.
However, having the pogo there means it's possible to completely skip it and play the rest of the game without it. Also, as Andy said, this level doesn't
have to be first.
If you look at it as an introductory level, I guess you could consider it a 'jump before you pogo' lesson. The player is rewarded for making it over the water with something that will probably make some other levels a lot easier.
The rayguns are excessive. I'd take a look at the other levels and consider whether that much firepower is really necessary. If they need to be there, I'd put one or two up in the top left, and another one a couple of tiles below the exit if necessary.
Points should not be in a place where they'll be collected as a matter of course. The hot dog, a high-point item, sits directly within the shortest path to the exit. The player would have to go out of their way to avoid it. I'd move it into the space above Keen's head, making sure it's out of reach from a normal jump and only accessible if the player decides to backtrack after collecting the pogo.
The bottles would go. I'd replace them with donuts hovering in the space between that single-tile platform and the ladder.
I'd be tempted to level the ground across the bottom slightly, either bringing up the lower sections by one tile, or bringing the middle section down one [but the height difference is a nice touch, so they'd still be offset].
Then the yorp needs attention. Placing tiles at both edges would make it feel cramped... maybe just one at the right so the player will at least interact with the yorp before it plummets to its doom [maybe it's just going swimming?]
The trees are too close to platforms for my liking... they need space. So I'd move the one in the bottom-right away from the ladder [or vice versa]... once the heights are adjusted the other one should be fine.
The size of the level is an integral part of it, IMKO. If it was made larger, it would cease to be this level.
Now, to put it through Vortlev...
I ended up taking a few more liberties here, most notably the two platforms along the top leading to the rayguns. There's also still the question of how much weaponry is required. The gun on the right stays, but the two on the left could be replaced with two bottles again.
The island in the middle should probably also have been extended one tile to the left, so there's more room for the yorp. The hot dog might then need to be shifted a bit, though. I also went against my decision to keep the heights offset, but the jutting bits at the edge of the island make up for that I think.
There should be some more points, too. I wasn't comfortable putting bottles in mid-air, so I used donuts. Perhaps I could've put a bottle on those two platforms and had the donuts hovering above the yorp.
This thread is turning out extremely well. There's so much interesting stuff to read, and for such a new topic there are already so many replies! Nice work, y'all!