I use a 3 leg puller from Supercheap to grip behind the splines. Put a thick cable tie around the legs and pull it tight so they don't slip out. As said the seal will come too.
I made a tapered alloy spline protector to fit the seal, but you can either wrap the splines in shim brass or use a stiff plastic bag pulled tight.
Easiest to fit seal to the gear as above, then slide gear onto crank. Then use a piece of PVC pipe and a block of wood to tap the seal in.
Black Rover MPI seals allegedly seal better than the red/orange ones.
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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.