first off the nextar is far from a performance plane,secondly at 6lbs which isn't really that heavy, and that giant wing you could putt it around the sky with a good 40 size engine as long as you had a long runway to take off from.
I totally understand not having a extra 20-40$ but the engine is the life blood of a airplane so skimping there is a bit of a mistake,if you have a OS 46LA then use that untill you can afford something better like a OS46AX,and if you dont have a engine yet check with your club, a member might have a decent engine you can use untill you get enough scratch to afford something nice for the future.
I have a fxi 46 that came with the RTF and a AX46 the AX has more grunt for sure and for 109.$ its a good price for what your getting, and will power any 40 size plane you can throw at it.
my Nextar has a OS 61 and floats putting a overly large engine on this airframe isn't going to help that much,unless your planning for the future if thats the case then get a 46-55AX after you save a few more $$, instead of spending less in the short term, get the best possable engine you can.this is just a floaty trainer not much of a stunter performance can come from your next planes after you master the basics