I still think plonking in a comeplete 4G93 engine would be better. More reliable, and u might end up spending about the same or just slightly more.
IF u were to REALLY wanna just plonk in a 1.8 block... you're going to have to purchase a complete block with pistons, conrods, everything inside the block lah! Coz' nothing in your 4G91 block u can use in your 4G93 block... as I've mentioned, the bore size is different. Extractor also u might have to change, coz' i think 4G91 extractor won't fit the now higher block. Then u gotta pay for installation which is quite expensive coz u're sorta like doing this semi-overhaul. NEw gasket set.
You'll also have to worry about your now 1.8 fueling... so have to go for rechip ECU or at least an SAFC. When all is done how much already man u've spent? Oh yes, not to mention you're probably going to have to get a 1.8 flywheel and clutch... coz' else ur 4G91's one might slip. Do take note that none of the removed items are hot items in the market, so even if u sell it cheap cheap, nobody might wanna buy.
Compression ratio might lari... and one of the more famous problems for this kinda conversion would be leaking black oil, or blowing the gasket.
Might as well go for complete 4G93, which is usually around 3k... plonk it in and sell off your 4G91... probably can compensate for the installation cash. So u'll just end up spending around 3k.
Worth it or not... u ownself think lah.