Thwimp's pretty simple. I made it for 2.0 a while ago but don't be discouraged as I think it's a pretty good place to start.
There's a tutorial on making an API here:
http://wohlsoft.ru/pgewiki/How_To:_Make ... custom_API
It only covers the fundamental basics of how an API is set up, but understanding this as the framework is pretty important.
If you're working with NPCs you have to be somewhat careful - their speedX and speedY cannot be modified if vanilla SMBX also modifies them. The workaround is:
x = x - speedX + customSpeedX (negating vanilla speed and adding your own). Here's some stuff NPCs can do:
http://wohlsoft.ru/pgewiki/NPC_(class)
Try to pick a simple NPC with not too much AI to negate. A spiny sounds ideal for the base for a thwimp.
You'll also wanna make use of pnpc to store variables like timers per npc object:
http://wohlsoft.ru/pgewiki/Pnpc.lua