Don't waste your money on a starter pack. Buy a multi-meter instead.
Your problem will be a massive voltage drop. One cause could be a battery which has lost it's "cranking amps" over the long storage. It might show 12-13 volts, but it will plummet when the starter is pressed. The other cause could be very high resistance in the starter cables.
The first course of action (and the cheapest) is to brighten up all the connections in the heavy starter circuit. There are three cables in this circuit; the thick ones about 1/4" or more diameter, so that's three ends to investigate. Your battery terminals are probably not too bad as you will have removed them to charge the battery, if not, do so. The worst one will likely be the one that bolts to the starter itself. Take care in removing this as the terminal post
must not turn . Don't ignore the earth lead going down to the front of the engine or to the inside of one of the footrest hanger bolts.
All the contact faces of the terminals need to be polished back to bright metal to ensure a really good connection. Use wire wool, emery or whatever.
This is a good service item in any case, so not a waste of time by any means.
If this shows no improvement then it's time for a new battery... Simples..
Cracked record alert!.. Are your timing belts over two years old? If so change them before trying to start it again as the 12 month lay up will have done them no good at all.