Aye Nell,
"I cannot get the engine to keep going long enough to give it a decent run, it's fine for several
minutes before the smoke starts then it cuts out."
To much oil?
Sticky oil pressure ball, sump filling up with oil because the return side of the pump isn't scavenging?
Any bleed screws to open to remove airlocks?
Old oil line internal collapse due to perishing or is there a lump of SH
TE in the return pipe?
Any internal strainers/mesh filters which might collapse and obstruct return to oil tank?
Never been in the guts of an RVFC, but I'd go from easy to get at, pull off, blow or poke through bits before dismantling.
Too much gasket goop or a gasket blocking a drilling? RTV etc stick like
to an army blanket and pattern gaskets don't always have the right holes in the right places or the right holes at all
If nothing obvious, get the gear on and go for a ride, even if it only manages 1 mile. Revving or ticking over with no load is not the same as riding the beast and applying load to all the parts. I am not saying you should thrash it, because it won't be up to working temperature, but it needs light work from the start. Your quote reads as though it is an accumalative malaise and even "several minutes" riding rather than waiting for the blue reek could make all the difference.
Sally (SRX600) after her rebuild got caught at lights after about 5 miles and after sitting ticking over for 2 minutes started producing clouds of blue smoke!
Stopped her, pushed her to the side of the road and let cool for 15 minutes. Started her up and the rode for another 20 miles with no stops to get home. No smoke and has been fine now for ~12K miles.
Please let us know what results you have.
All the best, Bill.