Well, I'm familiar with the older version Hobbico Avistar. Its a solid trainer with sport flying capabilities, but it's been discontinued. The newer version Great Planes Avistar is an updated version of the older one, also offering a trainer with sport flying cababilities. All of these planes mentioned are good candidates. With all of the trainers that Tower offers, you can view the manuals and check the prices for replacement parts like wings, fuselages and tail groups. The reason I mentioned the Avistar is because when I graduated from my Sig Kadet Senior, the Avistar is what I went to. Learned a heck of a lot from it. I'm even flying my second for sport flying now. The Tower 40 is ok too. Pretty economical, replacement parts are cheap. I'm not a fan of the landing gear, but it can be strengthened.
Your opening question was asking for help with your Magnum 46 xl. Have you solved that? If not, I suggest pulling it from the Nexstar and mounting it on a test stand. With the manual (which you can download
http://media.hobbypeople.net/manual/210756.pdf ) in hand, become familiar with starting and adjusting it. Have fun.