beedub-the versys trees are 50mm top and 52mm bottom. other forks may work.
the 650r twin weighs about 140#. not sure how that compares with the 650 honda thumper. there's the recent photos of the 650 twin motor in the xr600 frame-ebay listing.
he modified a few things to get it in there and it also looks like the engine is angled forward to clear at the top.