My HT5 engine starts with the pre-fix MJ... and frame RT..., when the frame was changed later by the factory it had the prefix CRT...
I do know of an Ariel works scrambles rider (Moto-cross to us post 1965ers) who used an HT frame on his works competition scrambles bike and said it handled well. The HT frame is a fine example of minimalism, single down tube and using engine as a stressed member.
As Adrie mentioned you would never have a sprung saddle on your frame but a nice comfy twin seat, a smaller competition version on the off road HS MK1 and full seat on the street scrambler HS MK3. As for the sidecar . . . unless it's really your thing, hide the damn thing in the loft and use the bike as nature intended, a lithe, responsive, high quality, single cylinder machine on green lanes etc. Probably will do that on my HT mainly as many 'modern' pre-65 trials seem to be set up for highly modified bikes and I don't want to smash it up too much!
Will be a nice bike though for sure!