They still make and sell the 9651!!
OP just FYI 9930 is just the keypad model number, thats the old type so tells us your alarm is at least 10 years old.
Where are you getting the manual from - online, or left with the house? Only the requirement for the 'zero' in front of the Eng code
went with the old shape keypad (more or less) so that won't help if you are trying to enter it and the system doesn't like it.
However, any installer and even DIY will have changed the engineer code from default or they need shooting.
Are you sure it's the 9851? Does it have a square metal case or an oblong one?
If you do indeed have the manual it will tell you how to default both codes without loosing the programming, but not a normal
DIY exercise and beware mains voltages in the box.
As above, probably needs a proper service and a new main battery.