iMac g3 motherboard replacement

No, not really. You'd have to make room for the Firewire ports on the newer motherboard.

Also there is no place for the VGA port on back that goes to the back slot.

Sonnet makes an equivalent upgrade designed for the iMac G3 233, but with one Firewire port.

You can do what I did, Get a 333 CPU card and replace the 233 card, will be a little faster, you could over-clock this 333 card to 400mhz with no stability problems.

The other thing you can do is max out the memory.

while not possible it is not that much more expensive to buy a used Slot Loading iMac than the cost of the motherboard alone.

the added abilities of the Slot Loading iMac will more than make up for the price difference.

Dear Hubert, the motherboards are I think a different size and shape. As someone has suggested, there will be little difference in buying this and buying a second-hand slot loader.
best wishes
Andrew Platt