Probably it's an early gearbox case- they needed a bit of grinding for a 1098 crank & rods to spin.
I once put an 1100S on an early 850 case, had to cut 4 friggin holes in front for the rods to miss!
Masking tape and araldite it, then paint- never leaked a drop!
re the SC gears- depends what they are and condition. New set of 3 synchro ones here is $1100+, 4 synchro ones $1600+.
Bit cheaper if brought in from UK.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.