Nice SP93. Dont worry about it being 'heavy' the relay transmission takes care of that by giving you dual variation. The stock AV7 comes with better breathing that the ones fitted to serie 40 and 50 Mobys so if you do swap it out for whatever keep the motor.
If you fit a AV10 to the frame you will need to fabricate lower mounts because the bottom of the motor bolts to the relay box.
Because of the relay box the engine counter rotates (backwards compared to other AV7) so if you go for CDI upgrade keep in mind that you will need to be clever with the timing, but same as other AV7-AV10, you would want 1,2mm BTDC with a CDI.