Depends on your carb skills or willingness to learn but for stock carbs stick with late model BS38's 78-79 or any year BS34's, the intake manifolds need to match the carbs. Engine intake, bolt up is same for all years. VM34 Mikunis are the most popular aftermarket carbs. You can buy them "jetted for the XS650" which will be at least a place to start from.
Stock "need rebuild" carbs can often be found for $50 or less, rebuilts $2-300 new VM's $300ish
Don't fall for the knock off VM's, while they may be OK there have been issues in the past.
Carbs from a Kawasaki twin are another popular "upgrade" but for a beach, bar hopper the BS34's would be my choice, easy to tune and forgiving of "close enough tuning"
Stock "need rebuild" carbs can often be found for $50 or less, rebuilts $2-300 new VM's $300ish
Don't fall for the knock off VM's, while they may be OK there have been issues in the past.
Carbs from a Kawasaki twin are another popular "upgrade" but for a beach, bar hopper the BS34's would be my choice, easy to tune and forgiving of "close enough tuning"