Definitely replace the timing gear.

The motor should not be nearly that difficult to turn (I assume you oiled the cylinders and the bearings as it went together). With Mahle pistons and rings, I can turn it easily by hand on the crank pulley until the point where the rocker arms go on.

Loosen the rod bolts so they are only finger tight. Does the motor turn easily now? If so, retorque the caps one at a time and see if all of them add drag, or just one. If it's all, you have incorrect bearing clearances; if only one, you have some dirt or a burr on that bearing shell. A very small imperfection here can make the motor very hard to turn.

If loosening the rod bolts does not improve the situation, check the ring gaps. There should be something on the order of 1/2 mm gap on each ring (although I think this would not bind up the motor on the stand unless there was NO gap).