Your OEM tires are getting pretty hard to find now..anyone with a set kicking around is likely holding them pretty close.
I suggest exploring the aftermarket..25 years of tire technology advance has done wonders for these old trikes.
You thought they handled well with the OEMs, you should see what you can do with some good replacements!!
Only time I personally go looking for anything with those soft riding OEM qualities is if the machine is a hard tail..
Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)