I am looking for recommandation for choosing the RTOS for MSP430 MCU with 256KB flash memory. My project is having few sensors, SD Card interfacing & few other switched / Leds. My current code size is 90kb+ without using any RTOS. The performance is the problem right now (tasks are not getting performed when require). So I want to switch to FreeRTOS / TI-RTOS. Also want to know, is it possible to migrate the current code into RTOS or have to re-write the complete code usign the RTOS.

These are very open questions that can only really have very long open answers. FreeRTOS support the MSP430 – if it is right for you depends on the application. I suggest starting by perusing the book – then get back with any specific questions. https://www.freertos.org/Documentation/RTOS_book.html