Hmmm... I've just built an A+ 1293 for my wife's car, it has twin HS4s, Tighe 104 cam, rimflow valves, unleaded seats, big port head, yada yada yada...
No it doesn't have a centre strap. But the rpm limit on this one will be 7000 (it has 21251 pistons in it).
I think a strap is overkill for the street.
Even on the track, I've been told by a certain race engine builder that centre caps only break if you use the engine to slow down and pull a zillion revs on overrun.
Yes you can do it at home, after you surface grind the cap flat. PM me for more info if you are determined to do it.
re the gearbox, I would strip it all out.
Check the following:
Wear on selector forks or levers
Wear on synchros (should have .030"+ clearance between ring & gear dogs)
Wear on nose of mainshaft (thru hardening)
End float of idler gear, & primary gear
Mainshaft and 1st motion shaft bearings- wear, noise, damaged cages
Replace:
Layshaft & needle bearings, mainshaft nose needle bearing
Idler gear bearings
Diff pin- replace pin with a competition one, and spider gears if worn inside. Also change the thrust washers inside diff.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
