Your rafter plan should be okay as drawn, as long as you use a board of adequate depth for the lower chord in your drawing. This would depend on the types of wood available and the expected snow loads in your area. You might ask a local builder or rafter company for their advice. You could use a board the same size as your rexisting rafter, but this might really be undersized and you wouldn't know it without checking
With all due respect, I wouldn't
use OSB for anything in structural construction. It has absolutely no strength-you can break it barehanded. If you aren't going to use structural connectors manufactured for the purpose, at least use 1/2" exterior grade plywood and galvanized construction screws. But Kevin45 is right, you want to brace both sides.
As for needing foot plates to distribute the load, you can figure the load by adding the weight of your lift and the weight of your vehicle and dividing that by the cross sectional area of your lift posts to get a pounds per square foot calculation. The are lots of tables for calculating the supporting strength of concrete. A call to your local concrete supplier could probably tell you if you need a wider support or not. Good luck.