Most shops, and some parts stores have a tester that mounts inplace of the rad cap that will test for the presence of hydrocarbons in your coolant indicating a head gasket. Is the white smoke all the time, or just when cold ?
There very well may be a problem with your head gasket, or it could be ok depending on what the test shows.
At any rate just wanted to address the fact that it won't have much more power if it is running in top shape.
My '86 Dolphin with the 22-RE and Auto Trans, struggles to maintain 60 mph, 55 up hills here in New England.
As much as we love these motorhomes, they are underpowered especially with the 4 cyl and the auto.
Sometime ago I had a '83 Huntsman with a 22-R and a 4 spd, and it had about the same power.
The EFI gives you a little more power, but any gain is lost in my Auto trans. That said I could get my Huntsman up to 65 Mph but it wouldn't stay there.
I've been debating putting a cam in my Dolphin to try to boost torque a little bit. I think if it had a little more, it may be able to maintain speed. It also feels like it is geared way too short. Sorry for the long winded response, but back to your last question.
Normally I would be very weary of using a product like a block seal, but if you have it tested and it is a head gasket, Your gonna have to pull the head anyway, (Don't forget the secret bolt in the front under the puddle of oil when pulling the head) so give it a shot.
Best of luck,
Justin