There's a lot of ways you could upgrade it, too many for me to list them all in detail. Let me tell you how I did mine. It isn't very fancy, but I'm happy with it.
I used two panes of glass I had sitting around from a previous setup, and fit nicely with an acceptable bit of overhang on the sides. Or you could get a fancier commercial one that fits perfectly, and includes a hinge and handle, like this:
With that, you can put any two
light strips on top.
So I placed the 17W T8 from the old hood on the back panel, and scored another cheap identical one (courtesy of Craigslist) for the front panel.
Finally, I slid the setup back a bit, so there's just enough of a gap to feed the fish without having to move or lift anything. I still have to move the front light to reach in for maintenance.
As a bonus, with light also shining from the front of the tank (not just from the back like most hoods of the type you have), the fish look better.
If you want other options, best to start a thread in the Lighting forum. More people will see and comment on it there. Mention that you want to stay low light, but want to grow Marsilea Minuta specifically. I haven't grown it myself, and it's possible it may need a bit more lighting still for best results.