Welcome to the Q&A Forum

You can definitely build your new mobile site with a different structure than your old OScommerce site, just make sure that your desktop site has an alternate tag pointing to your new site. I'd also recommend adding canonicals to your old mobile URLs pointing to the new versions; that'll allow you to keep the old mobile site alive, but stop Google from showing it in its index. Hope this helps! Kristina

I found the problem it was my sort function

Are these pages identical, but just reached via different paths (based on category/sub-category navigation)? If so, the canonical tag is definitely a good choice here. Ideally, it's best not to create duplicate paths, as it can cause long-term problems, but in lieu of that, canonical is probably your best defense.

in that situation, my recommendation would be a 302 to the next highest related page in the chain (such as the sub-category page or the category page above it), with the custom "we're sorry but that product is out of stock" message. If a product doesn't come back in stock over a relatively defined length of time (30 days, 60 days), etc., you should have the policy where the 302 becomes a 301 to that same page, with the same message.  It would mean that if a product DOES come back in stock after that, you'd cause some confusion from an SEO algorithm perspective, but it's the right thing to do.