You should check why the cable broke. If for some reason your tach is locked up it will promptly break the new cable too.
At the very least something squarish should be inserted into the tach head and spun to make sure it spins freely.
A bit of old cable will work but I have also used the squarish point of a nail or even a small phillips screwdriver.
BTW -- most towns have a speedometer shop that will install a new inner cable in your old housing for much less than the cost of a replacement cable. You just take them the old housing so they know how long to make the inner. Apart from less cost, this is desirable if your old housing is the original off-white color. Moss replacements are usually black, TRF is closer with a grey/silver colored replacement but I don;t think anything looks like the originals.