You might not have the hi/lo lever all the way engaged. Sometimes you have to rock the trike back and forth to get the lever to engage all the way. Also the chain may have some slop in it. What about the air filter? You may want to change a 20 some year old air filter for a new one, that might help your stumbling problem. Also when you changed your engine oil, did you remove your right side clutch cover and clean out you oil filter screen? Highly recommended also. Plus it won't hurt to change the oil again so soon on an engine that has set so long. Also if you used synthetic oil for your oil change, it is often too slippery for old clutches, so that also may be your clutch problem. Things to check. Also it would be a good idea to check your valve lash and give it a nice new spark plug and check your engine timing.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90