Paraphrasing from a very reliable source ...

Bearing removal. Rear: heat the crankcase around the rear main bearing 'til hot (use a glove or towel). With a solid downward motion, hit the rear of the block against a pad of paper or a piece of leather. The bearing will jar loose. Front: heat the crankcase around the bearing, tap the bearing out with a wood dowel and a plastic hammer.

Bearing installation. Freeze the bearings. Rc-bearings ship theirs in nice small ziplock bags - just toss it in the freezer. Rear: Slide the rear bearing down over the crankshaft 'til it bottoms out. If it won't slide down because the bearing is cold and the crank is warm - freeze the crank too. Heat the case around the bearing seat. Insert the bearing-n-crank with a wood dowel inserted into the back of the crank. Then seat the bearing into the case and use a sharp blow of a plastic hammer to seat if necessary. Front: heat the case around the front bearing seat. Have a solid surface with a hole drilled to accept the front of the crank. Place bearing over the crank and position the assembly so that the crank will go into the hole, then use the wood dowel inserted into the back of the crank and a plastic hammer drive the front bearing into its seat.

A reassembly note: the connecting rod big end hole is flat on one side and beveled on the other. When you reassemble, the bevel is installed towards the back plate.

Another approach:
slide the main bearing over the crank & seat it.
freeze both bearings & crank.
heat the case in the oven at 300 degrees.
insert the crank into the warm case - the bearing may seat beautifully - make sure that you keep the bearing seated while the hot & cold items equalize.
slide the front (frozen) bearing over the crank and it may seat itself.