I have owned and enjoyed a '69 1.1 GT since 1991...I seriously considered converting it to a 1.9 + and bought a wrecked '72 (?) to get all the parts.
After some good advice here from people I knew only thru this site I decided to restore my drive-able, rust-free 1.1. If I remember correctly
it was Rally Bob who said he wouldn't want a 1.1 ( No Surprise ! ) but that it was rare enough to preserve.. If you are reading this, Thank You Rally Bob
Later I bought two '73 GTs , one was a daily driver for a year or two. I never had enough time, money, and space at the same time to do a proper build.
But I did closely compare the '69 and '72 / '73 GTs. The body shells are basically identical. There are a bracket or 2 welded to the firewall for the 1.1 throttle
linkage which would be easy to remove... The sheet-metal at the opening quarter glass is slightly different also, but that has no effect on engine used...
Anyone thinking of going from a 1.1 to anything bigger or more powerful should take EVERYTHING mechanical off the body shell and save it for someone else.
Remove Complete Front Suspension, Springs, Brakes (everything is smaller) Radiator, Engine and Everything that attaches to it all the way to the rear wheels..
This obviously includes the motor mounts and cross-members, transmission, driveshaft, rear differential, ( which is relatively tiny, along with tiny rear brakes )
And some wires would need a fix because the alternator , sensors, etc are in a different location... But......You could use the same wheels !
Last week Todd at Opels Unlimited told me he had six used 1.1 engines. ( there were 2 (?) compression ratios available ) He is selling blocks and heads separately.
Someone in SoCal built a 1.1 Kadett for Lemons racing, starting with bigger valves.. Those are good little engines, I have a spare head and may explore the 'big valve' option.