Sounds like a fuel pump to me . Should hear a 'whirring' sound in the first couple of seconds coming from under rear seat(if all is quiet). How long has it been since fuel fiter changed ?
The same thing happened to me. I just used a center punch to loosen the bolts. It took a bit longer than I expected ! Then with a hacksaw cut a slot in the head to get a flat blade screwdriver to fit after cleaning my butchery up with a file(very hard steel). Then tap out the old key if you can...
And go up a wheelsize 13" to 14" > for clearance (if you have not gone to bigger wheels already ) N14 1.6l calipers & discs fit with a 1mm washer used as a spacer between calipers on the 2 x 17mm bolts roobarb.
Looks like turbo roller mower conversion is next ;) How long is that muffler! Looks like you've used a piece of zinc storm water downpipe. Are you going to stuff it with fiber glass or S/S wool. Allways thinking Frank ....
Your bell housing looks like Swiss cheese Frank :) Are you going to locate them by using the tabs in the water jacket? And how will you make a seal? Very cunning idea !