It's like a lot of things in life. Of course you could transplant your existing engine parts into the new cases, but the cost of gaskets will be immense, and probably more that getting a replacement engine. Don't even bother with pattern gaskets unless you want to do it all again! I'm not sure if you can do the job without completely taking the thing apart, I guess it depends on how the halves are bolted together. Maybe you could turn the engine upsidedown and just take the lower case off, but I don't know if you can get at all the fixings without taking the heads off, nor how this would affect locating the crank. It might be cheaper to see if the leak can be welded up.
As I'm bored at work, let's see what the potential cost of the job might be:
12391-mr8-000 Rocker cover gasket x2 £32.40 (only replace if they've gone brittle and are cracking)
12251-MR8-601 Head gasket x2 £68.40 (don't even think about trying to reuse or even use pattern ones!!)
11394-ml0-000 Clutch cover gasket £10.20
11636-ml0-000 Generator cover gasket £4.20
91302-mb0-013 Water pump o-ring £3.36
11632-ml7-000 (now 11632-MY7-000) Gearshift cover gasket £6.44
11315-ml0-000 Sump gasket (NC35 part is 11315-MR8-900, NC30 listed as NLA) £9.18
18291-MR8-000 Rear manifold to collector gasket x2 £5.04
18291-216-000 (now 18291-MN5-650) Exhaust gasket x4 (x2 if you don't need to remove the rear manifold) £6.16/12.34
So, that's roughly £145 before considering the odd bits that might also need replacing while you're at it. When taking the exhaust off, the packing between the front pipes and the collector may turn out to be no more than dust. The front cam cover bolts and rubber mounts tend to get very angry being nice and hot and having salty water thrown on them in winter, and general shite the rest of the year. Also, the clamps holding the pipes together like returning to nature. Aftermarket ones would be fine, but Honda prices are good for a laugh while we're making a nasty bill in Never Never Land! Oh, and while we're at it, exhaust studs have a habit of being pretty nasty, and so do the nuts.....
18391-310-000 x2 (now 18391-MN5-650) front pipe to collector packing £19.64
90543-ml0-720 x up to 8 Rubber seal for head cover £3 each
96001-06025-07 x up to 8 Flange bolt for head cover £0.90 each
18373-ml0-000 Right side exhaust clamp £18.60 (no, that's not a typo, they're almost twenty quid!)
18374-ml0-000 Left side exhaust clamp £17.70 (no, that's not a typo, Honda lefts and rights often cost different prices!)
90161-ml7-000 x2 Flange bolt for exhaust clamps (yes, that's right, almost forty quid for two clamps and the thieves charge extra for the bolts to tighten them!)
92900-06020-0E Exhaust stud x up to 8 £1.14 each
90304-hb3-771 Head stud nuts x up to 8 £2.70 each
90301-mr8-000 Exhaust to manifold stud nut x up to 2 £2.51 each
There's a reason it's costs over £40K to build an NC from spare parts from Honda...
and apon investigation, I found a hole in the lower crackcase just behind the starter motor 
. Initially I thought it might have been a gasket or or a loose crankcase bolt or something 
, but imagine my dissapointment when I found structural damage 
. Anyway, I managed to get my hands on another subassembly that is still in good nick 
.
lol. Not looking forward to a complete tear down, but if that's the only way, then ima have too 
. I do have a spare subassembly and I can use the crankcase from there and at a later stage have the other one repaired as spare.
