998 block had a 1000 badge on the side, and the engine number started with 9.
But as said the 998 & 1098 blocks of the same vintage are the same.. except for numbers.
People diss the 1098 but in stock form, it had 8lb/ft more torque and 10HP more than a common 998 did (not counting the Cooper).
IMO in a road car it's the next best thing to a 1275, as it goes up hills without rowing the gearlever.

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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.

Well, there are 12G heads and 12G heads...

For a 1100, you're probably best with a 12G202 (as I hear, anyway). That's probably what you've got already, as they came stock with the 1100s. Best thing to do would be to skim it down to give a better compression ratio (~10:1) and perhaps a little porting and polishing while you're at it.

Since it's off, you might as well change the valve guides to unleaded (unless they already have been). If you're going to do that, you might as well up the valve sizes a bit.

Yeah, I suppose it's about time and money as to how far you go. You can buy a big bore cylinder head (12G940?), but they're getting expensive, and will probably be more work than the 202.

12G295 is the best head for either a 998 or 1098.
It was used on 998 Cooper, Mk2A Sprite, MG1100 (sold in UK) and Morris 1100 automatic (not many made).
But, these heads are getting rarer and expensive. A 12G202 head can be ported and chambered to perform the same, but there's a fair bit of work involved.
The 202 head was stock on Morris 1100 and most 997 Cooper.

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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.

A sure fire way to tell which engine you have is to measure the stroke;
998= 76.2mm (3")
1098= 83.7mm (3.3")
Measure with 1 piston at TDC or add 1&2 together, you soon know what you've got.