Hi AnteP, I have a pair of 1998 GP800s. I've rebuilt the top and bottom engine on one, and the top on the other.

I agree with your plan. I would tear it apart an try to see why there's no compression. Compression that low I would think is usually a top end problem. Best case just some new gaskets. Piston/Rings? Any damage to cylinder walls? Could need new jugs. If so, and they are not too bad, you could have them "replated" by Millenium Technologies.
https://www.millennium-tech.net/

Or, just buy a some decent used ones off ebay. I've done both, and all good.

Be careful tearing things apart there are these little barrel dowel things the size of a skinny pencil eraser between the exhaust header & the jugs and between the jugs and the bottom end that can fall out when you take things apart if you're not careful. Don't lose them.

Getting pistons back into cylinder jugs is tricky, but simple with the right technique. If you're pulling out the engine anyway, closely inspect the bottom engine housing for any cracks/breaks.

SBT has a nice walkthrough on engine removal:
https://www.shopsbt.com/pdfs/y800.pdf
Definitely use the proper torque specs and blue loctite.

Another tip from someone else on these forums- put 1/2 inch stack of washers under the rear two bolts of the "ride plate" to keep the nose from bouncing/porpoise-ing.

Side note... I worked for a Swedish company some time ago and have fond memories of Goteborg.

My GP800's are super fun and nimble. Good luck with your replica!