Carbs for the 135 engines have been pretty short in the market just like the original CDI's have been, I however managed to grab a couple original 135 carbs in the beginning. they haven't really caused a problem for me uptil now so cant say which other carb works best, but I've heard DT125 Carbs can run the 135 engines pretty well, provided the right Jet kits are installed.
Your project sounds quiet interesting, no reason it shouldn't work out as planned. However, make sure the monoshock mounts are welded strong and straight enough, other than that everything seems right. Wouldn't mind seeing a few pictures of it once completed. If you need my help at any point down the road, I'd be glad to share whatever I know. Also, If you ever need a RX115 Genuine and straight chassis with clear papers, let me know as I have one lying in my garage. In any case, Good luck with the project Brother.