No, the carbs come off with the engine in place with ease. With the bodywork out of the way a long cross head screwdriver can reach the carb mount clips.

You say it's seized, when and how? Did it nip up sometime in the past? Is it still siezed now or can it be turned over?

But having sat for years the crankseals may also be suspect, you need Steve or Andy for mc18 crank help.
And as you say the carbs will need cleaning properly.

Anyway, a full engine rebuild is all in the workshop section (link on the left), it's an mc21 engine but the basics are the same, there's tonnes of info if you search the forums too if you can't find just ask!

And welcome to the forum.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread