Your Definately on the rite track with a montesa. As others have said there's a lot of fabricating involved and the best way is to make a subframe at the front and try to use existing bolts (i.e. engine bolts) so it can easily be put back to a solo without ruining the frame. I personally think the newer the bike the easier you will find it. I started with an old 1996 gas gas jtx320 and found that brilliant as it had so much grunt . I am now on a 2010 beta 290 and when I got that I found it so much easier. An old gas gas or montesa would be a perfect starting point. Where abouts are you from? If you can get to a local trial with sidecars that would be good as you could talk to people about there recommendations. We will all be willing to help and give advice.