You're correct with the calculation.
As you point out mass is the only variable and since mass is included only once then it is a linear relationship. Double the mass, double the force.
One thing to do to get a ballpark mass adjustment figure is to look at the primary reduction ration of the donor part bike and the primary for your bike and see what the difference in gearing is. Even at a 1500rpm engine idle speed the clutch shaft is spinning at a different rpm so the v2term will be different. You already have the force for your bike, now calculate the force for the bike the part is originally intended for and see how big the difference is and how much mass you need to change to get them to be equal.
Even after doing this the easiest thing to get a real world answer will be to throw it on the bike and test it as-is. If it works you're done. If its too weak you won't be able to go at low throttle. If its too strong you won't get the slippage at low speeds. Either way no damage is done and if it needs tweaking its behavior should confirm your calculations for more or less mass.