I own the Sig150 and as far as the internals go i'ts not badly built.
The fire button is very simple in that it's just a button that protrudes into the housing that presses up against the button on the board when you fire it. If you wanted to change this you would need to remove the board in order to carefully take out the stock button. Some models have the button ring just tightened on, I think others are tightened and help with adhesive, it can be a pain. I also think the ring is reverse threaded. Mine is very tight so I think it has the adhesive on the button ring.
The 510 connection is a little different in that it's "spring loaded" in the sense that a spring on the battery sled is pushing the 510 pin upwards. There is soldering on the pin and the negative of the 510.
I am by no means capable of installing a new 510 or a button, but I can tell you that it doesn't seem too difficult since the sled comes right out and the board seems to be held down with adhesive. However, I would not recommend tinkering with the internals unless you are fully capable of soldering wires back to the 510, removing the board carefully, and handy with tools to bore existing holes depending on what 510 and button you choose.
The Sig150 stock works as intended, the spring loaded 510 is a little odd, but if you wanted something more I would look into different box mods instead of modifying the 150. Unless you bought it at a steal and look at it as a project.