I want to know, any compatible brand which can replace the old link star modem with new modem. I meen I.e. Hughes or comtech modem will work on linkstar hub? Also want to know, at hub side what equipment we have to upgrade to do this replacement.

I think you can get second-hand and new Linkstar modems on ebay, but beware they might be 'locked' to some previous network.

As far as I know, Linkstar modems only work with a LinkStar hub, similarly Hughes modems only work with Hughes hubs. There was a plan to standardise the air interface but each manufacturer of VSAT seems to have added their own unique features.  I doubt hub staff could cope with a variety of remotes, each needing special software and skills to control.  Your comments please..

Comtech equipment comes as 19" rack mount style and is often operated point to point, with similar equipment at each end. TDMA is possible with multiple remotes sites working to a Comtech burst receiver at the hub and sharing a common outlink carrier. Both Star and Mesh networks are possible. Read more here: https://www.comtechefdata.com/products/satellite-modems

This data sheet https://www.comtechefdata.com/files/ds-cdm570A-L-IPEN.pdf refers to the Vipersat management system and dynamic SCPC carrier operation and bandwidth sharing on-demand.