FP can be made from a piece of steel rod of the correct diameter using an electric drill as a lathe. Spin the steel against a file to reduce the diameter until it is a good smooth fit in the hole in the bolt head. Adjust the protrusion.
The .303 barrel will require the chamber to be filled, with the filling piece protruding until it reaches the bolt face. It is taking up the space occupied by the .303 rim. Run the liner drill through the bore and the filler piece. I turn these to fit the chamber, others drill out the chamber and use a cylindrical filler. Bond the liner in, face off both ends, chamber & crown.
A .303 bolt head can be adapted. Solder the cut off firing pin to fill the hole. File the bolt face smooth. Locate and drill the smaller diameter hole through the bolt head. Reverse and drill the larger diameter hole deep enough. Lots of fiddling around, easy to mess up, start over.
Buying a .22 bolt head for $45 is worth it, unless you really want to convert a .303 head. The # of the bolt head doesn't matter - the breech protrusion is adjusted to reach the bolt face, and chambering sets headspace. Use a "0"; these are last desirable in .303. Don't pick a #3, these are useful if a .303 has excess headspace.
I don't think a .303 extractor is convertible. Maybe the end could be forged flat and recut. No. 7 and .22 SMLE extractors are close enough that either will work in either bolt head with a bit of fitting. Making an extractor is a possibility.
The extractor cut in the barrel breech takes a fair bit of careful filing. Files the right size are needed. Might be an idea to rework some to be safe edged. The cut must be carefully located. Probably have the barrel off and on a few times before the cut is correct.
Complete No. 7 rear sight sets are getting hard to find. Easy to use any of the No. 4 target sights.
The bayonet lugs can be turned off for a No. 7 appearance. Leave them on for a No. 9 look.
One of the LB .22 receivers can be used - or a .303 receiver. Only physical difference is at the right side rear sight mounting.
If using a 5/16" liner, it is not unusual to see traces of rifling left when the bore is drilled out. Doesn't matter as far as conversion goes, but says something about groove diameters.

