Saving Old Seagulls

Hi, have 3 queries I need help on.
I've replaced the head gasket on my s.century having done some work on it, but I've re-used the head bolts. Is this a mistake, should use new ones. If so, can I withdraw the old ones and replace with the new ones without disturbing the head gasket?
Also, on a s. century plus gearbox,I have a bolt going right through the water casing, in the middle next to the clutch rod and not a blind ended bolt. Would that be right? and if not , does it matter ?
I've joined a plus gearbox to my original s. century; so that I have a clutch, but I have since read that the s. century plus is a 5 hp motor and my present top end is 4hp. I thought they were the same cc engine, so are they different?
Thanks for your help. Red.

A copper gasket can be re-used if its in good condition, just anneal it and re-use.
Gearbox bolt sound right.
Yes, you can use the original head bolts.
What HP a seagull is quoted to be, is not necessary that HP it actually has.
I don't know about gearbox specs, try it and see.

Silver Century & Silver Century plus power heads, from the same production year have identical components & power outputs!

Thank you for your replies; you have answered all my queries. Much appreciated.
Red.