I like the concept, though had a difficult time with it. I think the early level has too many obstacles. Early on it would be nice if it was a bit easier to allow the player to experiment a bit more before needing to get thrusters and dealing with bumps and barriers. My first time I got stuck and had to remodel, one of my wheels went rolling off, which also limited my options for new designs.
I made it over to a bridge area that could be destroyed. When I fell down the screen zoomed out, and the game became very laggy. My character had also fallen onto its side, and with the lag I was not able to do much to try to right it. I clicked on the parts I could see, and clicked to remodel. Unfortunately, some of my parts fell behind the fallen bits of bridge and could not be recovered. I used what I had to try making a thruster based robot, but found it to want to tip over when using the thrusters. I had three, with one straight down, and one at each front and back with a slight, maybe 30 degree angle, away from the center, hoping it would offer some balance. Whenever I used the thrusters I would quickly tilt one way or the other and fall onto my side. With time and practice I could probably get it working, but the assembly could also use a bit of UI improvement. My biggest issue was not knowing where things would connect. I'd often drag part to where I want them, only to have them return to the top. I ended up generally connecting things in the middle to get them to stay, which was not an optimal location. I see that some pivot and tried using that, but while the initial gun mounted fine on the right side, I had trouble getting it onto the left side, even after rotating it. I think it would be nice if you added an indicator on any mounting points, and gave a brief description of how things connect. If the assembly was clearer I'd have tried more, as I like the idea a lot, but the large number of obstacle right at the start and not finding the assembly intuitive made it more frustrating than fun. I did still experiment with the parts I had and made the combined boxes, big and small, that I assume are tougher, as well as the upgraded thruster.